fix: improve error messages on cache init failures

.releaserc.yml

@@ -18,4 +18,4 @@ gradle:
       - '@semantic-release/release-notes-generator'
       - '@semantic-release/github'
   config:
-    registry: https://registry.npmjs.orgs/
+    registry: https://registry.npmjs.org/

src/main/groovy/de/illjut/gradle/semrel/SemrelPlugin.groovy

@@ -84,8 +84,13 @@ class SemrelPlugin implements Plugin<Project> {
 
       if (!cacheCompleteMarker.exists()) { // prepare cache for faster executions
         project.logger.info "preparing npm cache for faster executions."
-        nodeExec.executeNpm(['i', '--prefer-offline', '-D', "semantic-release@v${semanticReleaseVersion}".toString(), "@semantic-release/exec"], workDir)
-        cacheCompleteMarker.createNewFile()
+        def cacheResult = nodeExec.executeNpm(['i', '--prefer-offline', '-D', "semantic-release@v${semanticReleaseVersion}".toString(), "@semantic-release/exec"], workDir)
+
+        if (cacheResult.isSuccess()) {
+          cacheCompleteMarker.createNewFile()
+        } else {
+          throw new GradleScriptException("Failed to initialize NPM cache. See log for details.", new Exception("npm i failed. See details by running gradle with -i"))
+        }
       }
 
       def extraPackages = [ ]

src/main/java/de/illjut/gradle/semrel/ProcessResult.java

@@ -10,4 +10,8 @@ public ProcessResult(int exitCode, List<String> log) {
     this.exitCode = exitCode;
     this.log = log;
   }
+
+  public boolean isSuccess() {
+    return exitCode == 0;
+  }
 }