treat missing additional config files same as other missing config files

[jsmeix]

Treat missing additional config files (almost) same
as other missing config files which means that
missing additional config files do not let ReaR
abort with an Error but in contrast to other missing
config files missing additional config files are reported
to the user via 'LogPrint' so that the user is at least informed
if what he requested via the '-C' command line option cannot
be fulfilled.

An intended positive side-effect when missing additional
config files are no Error is that then it also works
(not relly cleanly but it works) for DRLM_MANAGED=y
which requires that missing local config files must
not let ReaR abort with an Error because the needed
config files get later downloaded from the DRLM server
and applied
(cf. the drlm_import_runtime_config function in lib/drlm-functions.sh)

For details see #1229