Before this change the link checker would ignore errors in a file if
the last link in a file was correct. The last link would wipe out the
error variable and set it to nil. Furthermore, it replaced errored
links with the empty string.

If we find an error that we can't correct, append the error message to
an an errs slice and leave the string as is.
Signed-off-by: Mike Danese <mikedanese@google.com>