Pull requests breaking when introducing changes to a submodule

Submodules in pull requests are not uncommon, but it appears if there is a change in a submodule or submodule has been removed from Source or Destination branch, or in certain conditions (renames of files + submodules) we throw a 500 error and the customer cannot view the diff nor merge the pull request using Bitbucket UI. 

Example of the error the end user might face is posted on the screen shot, you can view it from the Devtools console > network tab. 

The workaround in those situations: 

1. view the Diff locally and merge it locally and then push the update back to Bitbucket remote repository 
2. view the Diff locally and use API to merge the PRs

Also we should provide a better UI message to the end user to help them identify whey the error is coming up when trying to view the Diff for those PRs