Revision 1818964

Merge r1814968 from trunk:

core: silently ignore a not existent file path when IncludeOptional
     is used.

In https://bz.apache.org/bugzilla/show_bug.cgi?id=57585 some use cases
were reported in which IncludeOptional seems to be too strict in its
sanity checks.

This change is a proposal to relax IncludeOptional checks to silently
fail when a file path is not existent rather than returning SyntaxError.

Submitted by: elukey
Reviewed by: elukey, jim, niq