git-http-backend-verify: suppress git-http-backend errors
The git-http-backend routinely emits messages to standard error
about things that don't exist or requests that are not supported.
Unfortunately these errors end up cluttering up the server error
log and in almost all cases are uninteresting spam.
Discard these errors by default unless the environment variable
GIT_HTTP_BACKEND_SHOW_ERRORS is set to a non-empty value.
At the same time, make sure any "Internal Server Error" results
do end up in the server error log.