builtin/remote: quote remote name on error to display empty name

builtin/remote: quote remote name on error to display empty name

When adding new remote name with empty string, git will print the
following error message,

  fatal: '' is not a valid remote name\n

But when removing remote name with empty string as input, git shows the
empty string without quote,

  fatal: No such remote: \n

To make these error messages consistent, quote the name of the remote
that we tried and failed to find.

Signed-off-by: Shulhan <m.shulhan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>