Add more detailed stash error messages

Newly added in core commit I79bf0c29a4cef19363d111cc1128e35256ae572a,
shouldn't be any harm in merging this first.

Just gives us more information to deal with problems.

Change-Id: I78f76a1e92f767617ffd2c09f8cca5fa4d59f8b3
Merge "Add more detailed stash error messages"