Ensure beginning and end of the error output is readable

[kornelski]

When the build fails, the Cargo output is messy, and there's unhelpful noise added at both ends of openssl's message. The beginning of output gets a vague error and long meaningless stdout dump, and the end gets a backtrace of an unimportant code location.

This improves the output in two ways:

1. Emits cargo:warning= with the first paragraph of the error, so the cause of the failure is printed and highlighted by Cargo first,
2. Changes a panic to direct stderr print to avoid Rust adding unnecessary panic-related help.

[alex]

Do you have an example of a before-and-after of what this looks like?

[kornelski]

+The following warnings were emitted during compilation:
+
+warning: openssl-sys@0.9.103: Could not find directory of OpenSSL installation, and this `-sys` crate cannot proceed without this knowledge. If OpenSSL is installed and this crate had trouble finding it,  you can set the `OPENSSL_DIR` environment variable for the compilation process. See stderr section below for further information.
+
 error: failed to run custom build command for `openssl-sys v0.9.103 (~/rust-openssl/openssl-sys)`
 note: To improve backtraces for build dependencies, set the CARGO_PROFILE_DEV_BUILD_OVERRIDE_DEBUG=true environment variable to enable debug information generation.
 
 Caused by:
   process didn't exit successfully: `~/rust-openssl/target/debug/build/openssl-sys-fc33f8dce2cb8a74/build-script-main` (exit status: 101)
  --- stdout
   cargo:rustc-check-cfg=cfg(osslconf, values("OPENSSL_NO_OCB", "OPENSSL_NO_SM4", "OPENSSL_NO_SEED", "OPENSSL_NO_CHACHA", "OPENSSL_NO_CAST", "OPENSSL_NO_IDEA", "OPENSSL_NO_CAMELLIA", "OPENSSL_NO_RC4", "OPENSSL_NO_BF", "OPENSSL_NO_PSK", "OPENSSL_NO_DEPRECATED_3_0", "OPENSSL_NO_SCRYPT", "OPENSSL_NO_SM3", "OPENSSL_NO_RMD160", "OPENSSL_NO_EC2M", "OPENSSL_NO_OCSP", "OPENSSL_NO_CMS", "OPENSSL_NO_COMP", "OPENSSL_NO_SOCK", "OPENSSL_NO_STDIO", "OPENSSL_NO_EC", "OPENSSL_NO_SSL3_METHOD", "OPENSSL_NO_KRB5", "OPENSSL_NO_TLSEXT", "OPENSSL_NO_SRP", "OPENSSL_NO_RFC3779", "OPENSSL_NO_SHA", "OPENSSL_NO_NEXTPROTONEG", "OPENSSL_NO_ENGINE", "OPENSSL_NO_BUF_FREELISTS"))
   cargo:rustc-check-cfg=cfg(openssl)
@@ -59,12 +63,22 @@
   AARCH64_APPLE_DARWIN_OPENSSL_DIR unset
   cargo:rerun-if-env-changed=OPENSSL_DIR
   OPENSSL_DIR unset
-  run pkg_config fail: Failure { command: "\"pkg-config\" \"--libs\" \"--cflags\" \"opensslbork\"", output: Output { status: ExitStatus(unix_wait_status(256)), stdout: "", stderr: "Package opensslbork was not found in the pkg-config search path.\nPerhaps you should add the directory containing `opensslbork.pc'\nto the PKG_CONFIG_PATH environment variable\nNo package 'opensslbork' found\n" } }
 
+
+  Could not find openssl via pkg-config:
+  `"pkg-config" "--libs" "--cflags" "opensslbork"` did not exit successfully: exit status: 1
   --- stderr
-  thread 'main' panicked at openssl-sys/build/find_normal.rs:190:5:
+  Package opensslbork was not found in the pkg-config search path.
+  Perhaps you should add the directory containing `opensslbork.pc'
+  to the PKG_CONFIG_PATH environment variable
+  No package 'opensslbork' found
 
 
+  cargo:warning=Could not find directory of OpenSSL installation, and this `-sys` crate cannot proceed without this knowledge. If OpenSSL is installed and this crate had trouble finding it,  you can set the `OPENSSL_DIR` environment variable for the compilation process. See stderr section below for further information.
+
+  --- stderr
+
+
   Could not find directory of OpenSSL installation, and this `-sys` crate cannot
   proceed without this knowledge. If OpenSSL is installed and this crate had
   trouble finding it,  you can set the `OPENSSL_DIR` environment variable for the
@@ -81,19 +95,3 @@
   $TARGET = aarch64-apple-darwin
   openssl-sys = 0.9.103
 
-
-  stack backtrace:
-     0: rust_begin_unwind
-               at /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/std/src/panicking.rs:665:5
-     1: core::panicking::panic_fmt
-               at /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/core/src/panicking.rs:74:14
-     2: core::panicking::panic_display
-     3: build_script_main::find_normal::find_openssl_dir::panic_cold_display
-     4: build_script_main::find_normal::find_openssl_dir
-     5: build_script_main::find_normal::get_openssl::{{closure}}
-     6: core::option::Option<T>::unwrap_or_else
-     7: build_script_main::find_normal::get_openssl
-     8: build_script_main::find_openssl
-     9: build_script_main::main
-    10: core::ops::function::FnOnce::call_once
-  note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.