Add MERGE_FAILURE buildset result

We now report a distinct buildset result to the database if the upstream code
review system is unable to merge a change.  Previously it was reported as
MERGE_CONFLICT which makes it difficult to distinguish from merge conflicts.

Essentially, the two states we're interested in are when Zuul's merger is
unable to prepare a git checkout of the change (99% of the time, this is
a merge conflict).  This is the MERGE_CONFLICT result.

The second state is when Zuul asks Gerrit/Github/etc to submit/merge a change
and the remote system is unable (or refuses) to do so.  This is MERGE_FAILURE.

Change-Id: I47af242aeb19ae4b4aedde353fcc76ff82d90fbe

web/src/pages/Builds.jsx

@@ -83,6 +83,7 @@ class BuildsPage extends React.Component {
           'SKIPPED',
           'NODE_FAILURE',
           'MERGE_CONFLICT',
+          'MERGE_FAILURE',
           'CONFIG_ERROR',
           'TIMED_OUT',
           'CANCELED',

web/src/pages/Buildsets.jsx

@@ -71,6 +71,7 @@ class BuildsetsPage extends React.Component {
           'SUCCESS',
           'FAILURE',
           'MERGE_CONFLICT',
+          'MERGE_FAILURE',
           'DEQUEUED',
         ]
       },