compare tab is able to merge while in conflicted state

Description

I don't believe this is a regression for 1.6.0, but the Compare tab doesn't have the same restrictions as #6417 and I was able to get myself into a bit of a mess.

Version

• GitHub Desktop: 1.6.0-beta1
• Operating system: macOS

Steps to Reproduce

1. Clone this repository and add it to Desktop https://github.com/outofambit/blegh-test

2. With this repository active, switch to the History tab

2. Select branch as the compare branch

3. Attempt to merge

4. The merge fails unexpectedly (this is covered by #6101) and the merge conflicts popup is shown

5. Dismiss the popup, confirm the banner about conflicts is still shown

6. The Merge button is enabled, and the user can merge into the conflicted working directory

Expected Behavior

The Merge button should be disabled at this point because the index is marked as conflicted and nothing good will come from it. A tooltip on the disabled button should explain what is happening.

Actual Behavior

The default Git error handler shows a confusing message

Additional Information

N/A

Logs

2019-01-10T17:58:15.612Z - error: [ui] `git merge branch` exited with an unexpected code: 128.
error: Merging is not possible because you have unmerged files.
hint: Fix them up in the work tree, and then use 'git add/rm <file>'
hint: as appropriate to mark resolution and make a commit.
fatal: Exiting because of an unresolved conflict.