Fix up error messages

Since the error messages from the os package are self-describing, we don't need
to print everything twice.
mtime: Check for error return from Stat
Apply go fmt
Rename error to errExit

As it has the same name as a builtin type, using it will inevitably lead to
trouble like in tee.

Also, fix build of strings.
Change errExit to accept an error and print the command name
Merge pull request #3 from guelfey/master

Some small changes