WL#13423: Split errmgs-utf8.txt into one file for messages to clients and another for messages to the error log

The file errmsg-utf8.txt contains error messages:
a) messages from the server to clients (error code < 10,000)
b) messages for the error log

Within both of those two major groups, there exist
various sub-ranges (e.g. one per major server version,
so that when a message has to be backported, it can be
added at the end of that version's range where we have
some unused indices without which inserting that message
would renumber all that follow it).

Additionally, ranges can be reserved for special use.

Finally, each error-code can have multiple translations
of the same message.

This complexity makes it non-obvious where to add new
messages, how to backport a messages, and so on. A FAQ
has recently been added to the message file to cover the
most common and the most difficult cases, and to highlight
the fact that there is something to know in the first place
where this may not be obvious to the casual users (that is
to say, engineers not primarily concerned with the workings
of the error subsystems).

This changset splits the message file in two for the major
groups a) and b) as per above and uses obvious nomenclature
to further aid in this goal and to let us simplify the
included instructions (simplified and clarified instructions
also included in this patchset).

Finally, various failsafes are added to the utility that
processes these files to catch nonsensical directives,
messages in languages not previously declared (to identify
potential typos, or cases of different engineers using
different abbreviations for the same language), superfluous
directives (to identify potential misunderstandings), and
so on.

Approved by: Jens Even Blomsoy <jens.even.blomsoy@oracle.com>