[WIP] adding exit code to the preprocessor

[karimtera]

Description:

Adding exit code to the preprocessor, to allow continuing the preprocessing even after some errors are met.
So, the preprocessed output might contain a macro call that couldn't be expanded, or an include directive that file can't be found, that way the output would still be usable.

Exit code description:

It's a 4 bit code, with each bit shows if an error of a certain type is faced at least once or not.
First bit (rightmost significant bit): At least one System-Verilog file is not found.
2nd bit: At least one macro definition is not expanded.
3rd bit: At least one include file couldn't be found.
4-th bit is left for now.

[hzeller]

I've created an issue for this #1535