Fix the parsing functions: incomplete parses should not be accepted

This fixes an issue when parsing maxsat output: Yices prefixes the model output with a line specifying how many maxsat goals were not satisfied.
This confuses our parser which only parses this line (as garbage) and then gives up, ignoring the actual model specification.
With this change the parser will refuse to continue and error out, instead of returning a wrong value.
Filter maxsat info lines from the output.

I don't like this solution, it is flimsy. Yices could send all kinds of garbage to std output, and all this patch does is filter one instance of that.
I like better the approach of parsing line by line and then throwing away all the failed parses, but we cannot assume that every item of the model is going to fit in a single line.