gc: improve handling of errors reading gc.log
A collection of minor error handling fixes:
- use an error message in lower case, following the usual style
- quote filenames in error messages to make them easier to read and to
decrease translation load by matching other 'stat' error messages
- check for and report errors from 'read', too
- avoid being confused by a gc.log larger than INT_MAX bytes
Noticed by code inspection.
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>