Creates optional message for blockedKeyword

This makes it possible to add information to be presented when one
or more blockedKeyword(s) have a match.

To use it, add the following to the project.config:

[plugin "uploadvalidator"]
  rejectionMessage = message

Change-Id: I7c99db5bc0ec488feb6c13fb650b3878cd28759a

src/main/java/com/googlesource/gerrit/plugins/uploadvalidator/BlockedKeywordValidator.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/uploadvalidator/BlockedKeywordValidator.java b/src/main/java/com/googlesource/gerrit/plugins/uploadvalidator/BlockedKeywordValidator.java
index 41ad841..223f7c8 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/uploadvalidator/BlockedKeywordValidator.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/uploadvalidator/BlockedKeywordValidator.java
@@ -97,6 +97,10 @@
   private static final String KEY_CHECK_BLOCKED_KEYWORD_PATTERN =
       KEY_CHECK_BLOCKED_KEYWORD + "Pattern";
 
+  // This is used for a configuration option to append a custom message when blocked keywords are detected.
+  private static final String KEY_CHECK_BLOCKED_OPTIONAL_MESSAGE = "rejectionMessage";
+
+
   public static AbstractModule module() {
     return new AbstractModule() {
       @Override
@@ -176,8 +180,10 @@
                   cfg);
 
           if (!messages.isEmpty()) {
+            String rejectionMessage = cfg.getString(KEY_CHECK_BLOCKED_OPTIONAL_MESSAGE);
+            rejectionMessage = rejectionMessage == null ? "" : " " + rejectionMessage;
             throw new CommitValidationException(
-                "includes files containing blocked keywords", messages);
+                "includes files containing blocked keywords." + rejectionMessage, messages);
           }
         }
       }

src/main/resources/Documentation/config.md

diff --git a/src/main/resources/Documentation/config.md b/src/main/resources/Documentation/config.md
index 9c5f967..2d2701b 100644
--- a/src/main/resources/Documentation/config.md
+++ b/src/main/resources/Documentation/config.md
@@ -38,6 +38,7 @@
     allowedCommitterEmailPattern = .*gerrit\\.com
     allowedCommitterEmailPattern =  admin@gerrit\\..*
     rejectedCommitterEmailPattern = .*@old-name\\.com
+    rejectionMessage = yourMessage
 ```
 
 plugin.@PLUGIN@.allowedAuthorEmailPattern
@@ -451,3 +452,12 @@
     other skip rules.
 
     Default: false
+
+
+plugin.@PLUGIN@.rejectionMessage
+:   Additional message to be shown to user when blockedKeywords are found
+
+    The message is appended and printed as the final part of the message that
+    returns found Keywords.
+
+    Default: No message