Address leftover review comments

This commit addresses unresolved comments from the PR #611

Author: olafurpg

semanticdb-javac/src/main/java/com/sourcegraph/semanticdb_javac/NoRelativePathMode.java

@@ -1,5 +1,8 @@
 package com.sourcegraph.semanticdb_javac;
 
+import java.util.Arrays;
+import java.util.stream.Collectors;
+
 /**
  * Configuration options to determine how semanticdb-javac should handle files that have no good
  * relative paths.
@@ -24,5 +27,18 @@ public enum NoRelativePathMode {
   ERROR,
 
   /** Ignore the file, but print a message explaining it's ignored. */
-  WARNING
+  WARNING;
+
+  public static String validStringValuesWithoutError() {
+    return Arrays.stream(NoRelativePathMode.values())
+        .filter(mode -> !mode.equals(ERROR))
+        .map(NoRelativePathMode::toString)
+        .collect(Collectors.joining(", "));
+  }
+
+  public static String validStringValues() {
+    return Arrays.stream(NoRelativePathMode.values())
+        .map(NoRelativePathMode::toString)
+        .collect(Collectors.joining(", "));
+  }
 }

semanticdb-javac/src/main/java/com/sourcegraph/semanticdb_javac/SemanticdbJavacOptions.java

@@ -79,14 +79,10 @@ public static SemanticdbJavacOptions parse(String[] args, Context ctx) {
             result.noRelativePath = NoRelativePathMode.WARNING;
             break;
           default:
-            String validValues =
-                Arrays.stream(NoRelativePathMode.values())
-                    .map(NoRelativePathMode::toString)
-                    .collect(Collectors.joining(", "));
             result.errors.add(
                 String.format(
                     "unknown -no-relative-path mode '%s'. Valid values are %s.",
-                    value, validValues));
+                    value, NoRelativePathMode.validStringValues()));
         }
       } else if (arg.equals("-build-tool:bazel")) {
         result.uriScheme = UriScheme.BAZEL;

semanticdb-javac/src/main/java/com/sourcegraph/semanticdb_javac/SemanticdbTaskListener.java

@@ -222,12 +222,18 @@ private Result<Path, String> semanticdbOutputPath(SemanticdbJavacOptions options
         return null;
       case ERROR:
       default:
-        return Result.error(
+        String baseMessage =
             String.format(
                 "Unable to detect the relative path of '%s'. A common reason for this error is that the file is that this file is auto-generated. "
-                    + "To fix this problem, either configure the -sourceroot:PATH flag to be the parent directory of all indexed files, or "
-                    + "configure -no-relative-path:VALUE flag to have one of the following values: index_anyway, skip, warning.",
-                absolutePath));
+                    + "To fix this problem update the flag -no-relative-path:VALUE to have one of the following values: %s.",
+                absolutePath, NoRelativePathMode.validStringValuesWithoutError());
+        if (options.uriScheme == UriScheme.BAZEL) {
+          return Result.error(baseMessage);
+        }
+
+        return Result.error(
+            baseMessage
+                + " Alternatively, configure the -sourceroot:PATH flag to point to a directory path that is the parent of all indexed files.");
     }
   }
 }