fast-export: mark strings for translation

fast-export: mark strings for translation

Some error or warning messages in "builtin/fast-export.c" are marked
for translation, but many are not.

To be more consistent and provide a better experience to people using a
translated version, let's mark all the remaining error or warning
messages for translation.

While at it:

  - improve how some arguments to some error functions are indented,
  - remove "Error:" at the start of an error message,
  - downcase error and warning messages that start with an uppercase.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>