Calling evaluate() without a compiled expression returns NaN

[sylque]

Calling evaluate() without a compiled expression returns NaN. I think this doesn't make much sense. If it's forbidden to call evaluate() without a compiled expression, it should throw or assert.

[Blake-Madden]

I definitely should at least provide a decent error message. te_parser::get_last_error_message() just returns an empty string in this situation.

[Blake-Madden]

It will still return NaN (same as other invalid expressions), but a meaningful message will now be available via te_parser::get_last_error_message().