Make it possible to handle attach errors in key holder plugin.

[AlexPeshkoff]

In some cases (like encryption key entered by user in dialogue mode) it's very useful to be able to handle attach errors in key holder in order to be able to repeat with modified key value.

[AlexPeshkoff]

Solution ported from HQbird. Old key holders will proceed to work and compile as earlier.

[pavel-zotov]

@@@ QA issue @@@

It's probably impossible to do a test for this ticket.
But indirectly it can be checked by following actions:

1. add into firebird.conf parameter KeyHolderPlugin = fbSampleKeyHolder;
2. copy files fbSampleKeyHolder.conf & fbSampleKeyHolder.dll from %FB_HOME\examples\prebuilt\plugins\ to %FB_HOME\plugins\
3. run b/r of employee with specifying "-par " where <N> for sure more than number of logical CPUs, e.g.:
   %FB_HOME%\gbak -b -m -g localhost:employee stdout | %FB_HOME%\gbak -rep -v -par 99 stdin localhost:C:\FB\60sS\examples\empbuild\tmp.fdb -z 1>C:\TEMP\tmprestore.log.tmp 2>&1
4. get number of occurrences of string 'WARNING:' in the log:
   find /i /c "warning" C:\TEMP\tmprestore.log.tmp

Output of 'find' command must be:
------- C:\TEMP\tmprestore.log.tmp: 1

PS.
Problem initially was encountered when test tests/functional/tabloid/test_333be4bf.py started failing since 6.0.0.213 (15.01.2024).
(line with "WARNING:" missed there).

[AlexPeshkoff]

Rather useless test. It checks correct error handling in remote client. That has nothing to do with ability to handle errors in key holder plugin.