cmd/coordinator: add missing Sprintf arguments in gitMirrorErrors
cmd/coordinator: add missing Sprintf arguments in gitMirrorErrors

This is a followup to CL 179877 that adds missing arguments to
a fmt.Sprintf call.

Also change the if condition to ensure the regexp had exactly 3
matches as expected. That makes it more clear there won't be
a panic when accessing matches.

Change-Id: Ibf57d87565c8297cd68dd9a63b4dc709bce613de
Reviewed-on: https://go-review.googlesource.com/c/build/+/182017
Run-TryBot: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Emmanuel Odeke <277f8eaa8c7dfbc55c17bf5e30561c1a9c45c569@gmail.com>

golang/build,golang/build,golang/build,golang/build