MINOR: config: improve .if condition error reporting

MINOR: config: improve .if condition error reporting

Let's return the position of the first unparsable character on error,
so that instead of just saying "unparsable conditional expression blah"
we can have:

  [ALERT] 125/150618 (13995) : parsing [test-conds2.cfg:1]: unparsable conditional expression '12/blah' in '.if' at position 1:
    .if 12/blah
        ^
This is important because conditions will be made from environment
variables or later from more complex expressions where the error will
not always be easy to locate.