ghc 8 added backtraces on uncaught errors. This is great, but git-annex was
using error in many places for a error message targeted at the user, in
some known problem case. A backtrace only confuses such a message, so omit it.
Notably, commands like git annex drop that failed due to eg, numcopies,
used to use error, so had a backtrace.
This commit was sponsored by Ethan Aubin.
This was potentially a hole in the readonly mode armor even before my last
commit. If the user could push a git-annex branch to a repo, they could get
git-annex-shell to initialize the repo. After my last commit, the user
didn't even need to be allowed to push a branch to init the repo, so
this hole certianly needs to be closed now.
