Use custom error dialogs for all Git errors

[ianhi]

Description

follow-up of #619

The jupyterlab provided showErrorMessage is nice because it is consistent with the rest of the application and is always available. However I think that it is generally not expressive enough for showing git related error messages. These error messages generally have some structure to them when they are present by the git CLI, for example git pull:

but with showErrorMessage it is impossible to retain this structure and the message ends up all squashed together:

The other case where this comes up is listing files that have merge conflicts. In that case it is nice to visually separate the list of files from the rest of the error message.

Would it be beneficial to have a custom error message template for all these case, or is it fine to fix them all individually using showDialog?

List of error dialogs that could be improved:

Push and pull:

Failed pull:

could offer a button in the dialog to run the set-upstream-to command
push:

vs:

Unlike pull this is an edited version of the error text that git generates. Would be nice to prompt that a remote needs to be added.

revert commit failure:

failed clone:
I think the 'fatal' isn't helpful in the context of an error message, if anything it could add confusion

[kgryte]

In general, I'd argue that we should ditch all JLab dialogs in the Git extension. They are harder to work with compared to React dialogs, and they introduce a separate coding paradigm within the codebase.

[ianhi]

It maybe will be helpful to have a catalog of the various errors that can can result from failed git commands. It's not always obvious what all the failure modes of a git command are and they will often require different formatting for nice display. If there are no complaints I'll post errors as I encounter them here? Also this error from a failed revert commit (have staged changes that will conflict with the reversion of an old commit) is spectacular - "Error: error: ...."

With the same changes but without staging them: