Merge conflict error messages and handling

[achilleas-k]

This PR makes some minor changes to how merge conflicts are detected and handled on gin download.

• In the case where a download fails in order to avoid overwriting locally modified or untracked files (download failed: local modified or untracked files would be overwritten by download), the files in question are listed after the error message.
• In the case where a download fails because git files can't be merged, a new error message is printed download failed: files changed locally and remotely and cannot be automatically merged (merge conflict) and the files in question are listed after the error message.
• In the case where a download fails because annex files can't be merged, a new error message is printed files changed locally and remotely. Both files have been kept and the files in question are listed after the error message.

In the latter case, conflicting files are kept in the local directory and added automatically by git annex (with a unique suffix).

[achilleas-k]

With the new commits, the client now checks for merge conflicts even when the download (git annex sync) command succeeds, because git-annex will return with a successful exit status (0) when it resolves a conflict by keeping both annex variants of a file. We want to catch this and return with an error to notify the user that something out of the ordinary has happened and point them to the conflicting files which require attention.

This also makes combinations of conflicts (overwrite warnings + git files conflicting, or git + annex files conflicting, etc) behave in the safest way.