Report meaningful message when both --enable_workspace and --enable_b…

…zlmod are false

WorkspaceNameFunction is the first one to fail when those two flags are off.

Working towards: #23023

PiperOrigin-RevId: 656006850
Change-Id: I4f7dda4cc3c2b19085d22490fd457fc53f267ed6

src/main/java/com/google/devtools/build/lib/skyframe/WorkspaceNameFunction.java

@@ -67,17 +67,19 @@ public SkyValue compute(SkyKey skyKey, Environment env)
       }
       Package externalPackage = externalPackageValue.getPackage();
       if (externalPackage.containsErrors()) {
-        throw new WorkspaceNameFunctionException();
+        throw new WorkspaceNameFunctionException("Failed to load the WORKSPACE file");
       }
       return WorkspaceNameValue.withName(externalPackage.getWorkspaceName());
     }
-    throw new WorkspaceNameFunctionException();
+    throw new WorkspaceNameFunctionException(
+        "Both --enable_bzlmod and --enable_workspace are disabled, but one of them must be enabled"
+            + " to fetch external dependencies.");
   }
 
   private static class WorkspaceNameFunctionException extends SkyFunctionException {
-    WorkspaceNameFunctionException() {
+    WorkspaceNameFunctionException(String message) {
       super(
-          new BuildFileContainsErrorsException(LabelConstants.EXTERNAL_PACKAGE_IDENTIFIER),
+          new BuildFileContainsErrorsException(LabelConstants.EXTERNAL_PACKAGE_IDENTIFIER, message),
           Transience.PERSISTENT);
     }
   }