royale-maven-plugin: more specific error message for exit code from --watch compiler option

Using --watch might be allowed in the future, but for now, we should explain that it doesn't work
diff --git a/royale-maven-plugin/src/main/java/org/apache/royale/maven/BaseMojo.java b/royale-maven-plugin/src/main/java/org/apache/royale/maven/BaseMojo.java
index 26ff794..8dc2656 100644
--- a/royale-maven-plugin/src/main/java/org/apache/royale/maven/BaseMojo.java
+++ b/royale-maven-plugin/src/main/java/org/apache/royale/maven/BaseMojo.java
@@ -385,6 +385,9 @@
     }
 
     protected void handleExitCode(int exitCode) throws MojoExecutionException {
+        if(exitCode == 1000) {
+            throw new MojoExecutionException("The --watch compiler option is not supported by royale-maven-plugin");
+        }
         if(exitCode != 0) {
             throw new MojoExecutionException("There were errors during the build. Got return code " + exitCode);
         }