Adjust error message when Extending something that's not a Type/Union

It should mention Unions too, not just Types.
symbFindClosestOvlProc(): Replace iif() with OR
hAddOvlProc(): Simplify property check
parser: Fix error recovery in cAnonType()

cCtorCall() can return NULL and several other places of code check for it,
but cAnonType() didn't. It shouldn't return NULL though, because its
callers don't handle it (specifically: cStrIdxOrMemberDeref(), see also
02c411ce). Instead, it should return a CONST or TYPEINI; the new call to
astNewCONSTz() takes care of that.
parser: Improve error recovery in cCtorCall()

If cCtorCall() saw a '(', it should also try to parse the corresponding ')'
even if cProgArgList() failed. This prevents "expected EOL, found ')'"
errors later.
ast: Adjust astNewCONSTz() to return NULL for FB_DATATYPE_VOID

Previously it would do an astNewCONSTi() where the astNewCONV() (added in
ea511505 on the 64bit branch) failed to convert the integer to VOID.