Add a link to syntax.md after error messages

* Example error messages from the upload validator:
    remote: ERROR: commit ...: syntax: OWNERS:3: ...
    remote: ERROR: commit ...: ignored: OWNERS:5: ...
    remote: ERROR: commit ...: unknown: xyz@g.com at OWNERS:2
    remote: ERROR: commit ...: See OWNERS file syntax document
    at https://gerrit.googlesource.com/.../syntax.md

Change-Id: I8f58e0237a5de542a82176187ea72e5ffd0cbee8

src/main/java/com/googlesource/gerrit/plugins/findowners/OwnersValidator.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/findowners/OwnersValidator.java b/src/main/java/com/googlesource/gerrit/plugins/findowners/OwnersValidator.java
index deda93e..5c6e050 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/findowners/OwnersValidator.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/findowners/OwnersValidator.java
@@ -135,6 +135,9 @@
       throw new CommitValidationException("failed to check owners files", e);
     }
     if (hasError(messages)) {
+      add(messages, "See OWNERS file syntax document at "
+          + "https://gerrit.googlesource.com/plugins/find-owners/+/"
+          + "master/src/main/resources/Documentation/syntax.md", true);
       throw new CommitValidationException("found invalid owners file", messages);
     }
     return messages;