Do not silence failure form subtree command

[simensen]

@igorw had an issue that would have been more easily detected if errors form subtree were not silenced.

The reasons for the silence to begin with was because prefixes may legitimately not exist in all branches and the output would have been somewhat confusing/meaningless:

'src/MyProject/Missing' does not exist; use 'git subtree add'

Suggestion was to try and catch the error and do something different depending on the response. For example, search output for "git subtree add" and if it exists, ignore it, otherwise, fail and show it (or at least warn somehow).

[skyzyx]

👍