Fix crash on bazel mod error containing %

`bazel mod show_extension @foo//:extensions.bzl%go_sdk`resulted in the crash:
```
java.util.IllegalFormatConversionException: g != java.lang.String
```

Also makes an error more readable by swapping a `:`.

Closes #20627.

PiperOrigin-RevId: 592942775
Change-Id: Ida5c234413c1647f81d3702bb9deeedcdd93df12

src/main/java/com/google/devtools/build/lib/bazel/commands/ModCommand.java

@@ -278,7 +278,7 @@ private BlazeCommandResult execInternal(CommandEnvironment env, OptionsParsingRe
                           baseModule.getUnusedDeps()));
             } catch (InvalidArgumentException | OptionsParsingException e) {
               throw new InvalidArgumentException(
-                  String.format("In extension argument: %s %s", arg, e.getMessage()),
+                  String.format("In extension argument %s: %s", arg, e.getMessage()),
                   Code.INVALID_ARGUMENTS,
                   e);
             }
@@ -494,13 +494,10 @@ private static ImmutableSortedSet<ModuleExtensionId> extensionArgListToIds(
 
   private static BlazeCommandResult reportAndCreateFailureResult(
       CommandEnvironment env, String message, Code detailedCode) {
-    if (message.charAt(message.length() - 1) != '.') {
-      message = message.concat(".");
-    }
     String fullMessage =
         String.format(
-            message.concat(" Type '%s help mod' for syntax and help."),
-            env.getRuntime().getProductName());
+            "%s%s Type '%s help mod' for syntax and help.",
+            message, message.endsWith(".") ? "" : ".", env.getRuntime().getProductName());
     env.getReporter().handle(Event.error(fullMessage));
     return createFailureResult(fullMessage, detailedCode);
   }