Improve rebase fail message

When rebasing on an existing commit that is not on the destination branch,
the error message was "base revision is missing" which was not actually true.
Improve the message to "base revision is missing from the destination branch".

Change-Id: I51742a20458169cd3b108eb54f4371eb64d65844
diff --git a/java/com/google/gerrit/server/restapi/change/Rebase.java b/java/com/google/gerrit/server/restapi/change/Rebase.java
index 2e2f565..eed0896 100644
--- a/java/com/google/gerrit/server/restapi/change/Rebase.java
+++ b/java/com/google/gerrit/server/restapi/change/Rebase.java
@@ -161,7 +161,8 @@
 
     Base base = rebaseUtil.parseBase(rsrc, str);
     if (base == null) {
-      throw new ResourceConflictException("base revision is missing: " + str);
+      throw new ResourceConflictException(
+          "base revision is missing from the destination branch: " + str);
     }
     PatchSet.Id baseId = base.patchSet().getId();
     if (change.getId().equals(baseId.getParentKey())) {