Handle errors in get-master/create-branch/add-file/create-pull-request process

[ckingbailey]

In all cases, clean up after myself if process failed.

• Delete created branch

Possible errors

From create-branch endpoint

422 Unprocessable Entity

"message": "Reference already exists",
"documentation_url": "https://developer.github.com/v3/git/refs/#create-a-reference"

From get-master endpoint

404 Not Found: Master branch does not exist

When creating new file via the content endpoint

InvalidCharacterError thrown by window.btoa() when users enters non-ASCII character in any field.