If no default editor found, user cannot use Merge Conflicts

If no default editor found, Desktop prompts the user to download Atom, thus disrupting the Merge Conflict flow as other git actions need to be resolved before continuing with Merge Conflict

Alternatively, I have seen this error too:

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.

Reproduce:

• Remove your editors, restart Desktop
• Attempt a merge conflict, click open in editor button

Logs:

2018-10-31T17:50:55.535Z - info: [ui] launching: 1.5.0-beta2 (Mac OS 10.13.6)
2018-10-31T17:50:55.536Z - info: [ui] execPath: '/Applications/Beta Desktop.app/Contents/Frameworks/GitHub Desktop Helper.app/Contents/MacOS/GitHub Desktop Helper'
2018-10-31T17:51:25.404Z - info: [ui] [AppStore.withAuthenticatingUser] account found for repository: PRlist - tierninho (has token)
2018-10-31T17:52:03.465Z - error: [ui] `git merge origin/ccc` exited with an unexpected code: 1.
Auto-merging .gitignore
CONFLICT (content): Merge conflict in .gitignore
Automatic merge failed; fix conflicts and then commit the result.

(The error was parsed as 7: We found some conflicts while trying to merge. Please resolve the conflicts and commit the changes.)
2018-10-31T17:52:12.709Z - info: [ui] [AppStore.withAuthenticatingUser] account found for repository: PRlist - tierninho (has token)
2018-10-31T17:52:12.732Z - error: [ui] `git -c credential.helper= checkout --progress origin/ccc -b ccc --recurse-submodules --` exited with an unexpected code: 1.
.gitignore: needs merge

error: you need to resolve your current index first

2018-10-31T17:52:56.924Z - info: [ui] [AppStore.withAuthenticatingUser] account found for repository: NEWtest - tierninho (has token)
2018-10-31T17:52:57.979Z - info: [ui] Executing fetch: git -c credential.helper= fetch --progress --prune --recurse-submodules=on-demand origin (took 1.048s)
2018-10-31T17:52:58.377Z - info: [ui] [AppStore.withAuthenticatingUser] account found for repository: asdasdasddsdss - tierninho (has token)
2018-10-31T17:52:59.937Z - info: [ui] Background fetch for 12 repositories took 4.745sec
2018-10-31T17:53:55.816Z - info: [ui] [AppStore.withAuthenticatingUser] account found for repository: PRlist - tierninho (has token)
2018-10-31T17:53:55.837Z - error: [ui] `git -c credential.helper= checkout --progress origin/ccc -b ccc --recurse-submodules --` exited with an unexpected code: 1.
.gitignore: needs merge

error: you need to resolve your current index first

2018-10-31T17:54:46.240Z - error: [ui] `git merge origin/ccc` exited with an unexpected code: 128.
error: Your local changes to the following files would be overwritten by merge:
	.gitignore
Please commit your changes or stash them before you merge.
Aborting

tested in: 1.5.0-beta2, mac

[ampinsk]

We should at least jump back to the conflict resolution modal if they hit Close on the “Unable to open external editor” popup.

I like giving people the prompt to download Atom, but maybe it would be safer to do something like disable the Open in Editor button with a tooltip No editor detected?

[shiftkey]

Would it be possible to do something like Open in... and have them choose an application?

@ampinsk as a fallback we have the “Open in Default Program” context menu action in some areas, but I don’t think it’s ready to be relied on for our features for a couple of reasons:

• this defers to whatever the OS has registered for an extension, and we have no way to tell whether a program is registered for this extension - see https://github.com/desktop/desktop/issues/5411 for tracking this problem
• each platform has it’s own quirks about the underlying implementation - https://github.com/desktop/desktop/issues/4753#issuecomment-392394769 has some details about how us using this API lead to confusion for Windows users when their expected program doesn’t launch