branch: report errors in tracking branch setup

branch: report errors in tracking branch setup

When setting up a new tracking branch fails due to issues with
the configuration file we do not report any errors to the user
and pretend setting the tracking branch succeeded.

Setting up the tracking branch is handled by the
`install_branch_config` function. We do not want to simply die
there as the function is not only invoked when explicitly setting
upstream information with `git branch --set-upstream-to=`, but
also by `git push --set-upstream` and `git clone`. While it is
reasonable to die in the explict first case, we would lose
information in the latter two cases, so we only print the error
message but continue the program as usual.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>