#475 reword error messages according to standard

[Simon Sobisch]

This is kind of an ongoing process I recently was reminded about. If someone spots entries that are not "correct", please drop a note here. The goal is to be able to easily find references in the standard when looking for msgids output by cobc and also internally during development of GnuCOBOL.

The changes for msgids should always result in necessary testsuite adjustments (otherwise we have a missing testcase to add as well).

First entry: "PICTURE clause" (we have that right), but "picture symbol" (we upper-case that currently) and "picture character-string" (we use several versions of "picture string" currently and in some places should switch to picture symbol).