Skip Sentry configuration in Lando environments

** Why are these changes being introduced:

We integrate with Sentry provided a given ENV value is defined in
secrets.json, but this can result in local environments sending
exceptions to Sentry - which be quite noisy and undesired.

** Relevant ticket(s):

* https://mitlibraries.atlassian.net/browse/pw-106

** How does this address that need:

This adds an additional check around the Sentry configuration block,
making sure that the PANTHEON_ENVIRONMENT variable is not sent to
"lando" when integrating with Sentry.

** Document any side effects to this change:

None

docs/howto/troubleshoot-build-failures.md

@@ -0,0 +1,29 @@
+# How to troubleshoot a build failure on Pantheon
+
+This document describes some helpful steps to take when you run into a build
+failure on Pantheon. These failures are seen most often when creating a brand
+new instance.
+
+## Identifying a build failure
+
+A build failure on Pantheon might become evident when creating a multidev initially:
+
+```bash
+% terminus multidev:create mitlib-wp-network.live wp66
+ [error]  The operation failed to complete. 
+```
+
+It might also appear when trying to clone content between tiers:
+
+```bash
+ % terminus remote:wp mitlib-wp-network.wp66 -- search-replace libraries.mit.edu wp66-mitlib-wp-network.pantheonsite.io --url=libraries.mit.edu
+ [warning] This environment is in read-only Git mode. If you want to make changes to the codebase of this site (e.g. updating modules or plugins), you will need to toggle into read/write SFTP mode first.
+Warning: Permanently added 'REACTED' (RSA) to the list of known hosts.
+Enter passphrase for key 'REDACTED': 
+Error: This does not seem to be a WordPress installation.
+The used path is: /code/web/wp/
+Pass --path=`path/to/wordpress` or run `wp core download`.
+ [notice] Command: mitlib-wp-network.wp66 -- wp search-replace libraries.mit.edu wp66-mitlib-wp-network.pantheonsite.io [Exit: 1]
+ [error]
+```
+

web/wp-config.php

@@ -66,7 +66,7 @@
     define( 'WPMS_SMTP_PASS', $secrets['WPMS_SMTP_PASS'] );
 
     // Sentry configuration is optional.
-    if ( array_key_exists( 'SENTRY_DSN', $secrets ) ) {
+    if ( array_key_exists( 'SENTRY_DSN', $secrets ) && $_ENV['PANTHEON_ENVIRONMENT'] != 'lando' ) {
       define( 'WP_SENTRY_DSN', $secrets['SENTRY_DSN'] );
       define( 'WP_SENTRY_ERROR_TYPES', E_ERROR & E_CORE_ERROR & E_COMPILE_ERROR );
       define( 'WP_SENTRY_VERSION', 'v1' );