[FIXED JENKINS-14262] Add more diagnostic detail to failure message.

src/main/java/hudson/plugins/copyartifact/CopyArtifact.java

@@ -232,7 +232,7 @@ private boolean perform(Run src, AbstractBuild<?,?> dst, String expandedFilter,
                                 : new FilePath(src.getArtifactsDir());
         if (srcDir == null || !srcDir.exists()) {
             console.println(useWs ? Messages.CopyArtifact_MissingSrcWorkspace() // (see JENKINS-3330)
-                                  : Messages.CopyArtifact_MissingSrcArtifacts());
+                                  : Messages.CopyArtifact_MissingSrcArtifacts(srcDir));
             return isOptional();  // Fail build unless copy is optional
         }
 

src/main/resources/hudson/plugins/copyartifact/Messages.properties

@@ -7,7 +7,7 @@ CopyArtifact.MissingBuild=Unable to find a build for artifact copy from: {0}
 CopyArtifact.MissingProject=Unable to find project for artifact copy: {0}\n\
 This may be due to incorrect project name or permission settings; \
 see help for project name in job configuration.
-CopyArtifact.MissingSrcArtifacts=Unable to access upstream artifacts area. Does source project archive artifacts?
+CopyArtifact.MissingSrcArtifacts=Unable to access upstream artifacts area {0}. Does source project archive artifacts?
 CopyArtifact.MissingSrcWorkspace=Unable to access upstream workspace for artifact copy. Slave node offline?
 CopyArtifact.MissingWorkspace=Unable to access workspace for artifact copy. Slave node offline?
 CopyArtifact.ParameterizedName=Value references a build parameter, so it cannot be validated.