merge-ort: convert more error() cases to path_msg()

merge-ort: convert more error() cases to path_msg()

merge_submodule() stores errors using path_msg(), whereas other call
sites make use of the error() function.  This is inconsistent, and
moving towards path_msg() seems more friendly for libification efforts
since it will allow the caller to determine whether the error messages
need to be printed.

Note that this deferred handling of error messages changes the error
message in a recursive merge from
  error: failed to execute internal merge
to
  From inner merge:  error: failed to execute internal merge
which provides a little more information about the error which may be
useful.  Since the recursive merge strategy still only shows the older
error, we had to adjust the new testcase introduced a few commits ago to
just search for the older message somewhere in the output.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>