[Bitbucket]: PR merge failure error pop up description

**Is your feature request related to a problem? Please describe.**
Merging a pull request has requirements such as: passing build status

The extension throws an error but does not give specific details of the reason the merge failed.

`Merging the pull request has been vetoed` is very generic.


**Describe the solution you'd like**
Provide a clear indication to the user why the merge has failed. Prevent `merge` button from action until all PR merge requirements are satisfied.

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.

**Additional context**
Add any other context or screenshots about the feature request here.

Start by tracing the Bitbucket pull-request merge action and the error pop-up shown by the VS Code extension. Done means the merge failure explains the unmet requirement and the merge button cannot be used until all pull-request merge requirements are satisfied.

Written by the indexing model from the issue text.