Added exception handling for Props(FluidName, Output) method

The internal `_Props1` method for constants has thrown an `ValueError` on wrong inputs without exception handling, which led to a termination of the program. Thus, on Python the `testBadParam` test failed.

I have added a try/except clause to the calling `Props` method and also moved the existing exception handling for REFPROP fluids from the internal `_Props1` method outside to the caller. To avoid duplication of the exception handling code, I have  redirected calls to `Props(char *FluidName, char *Output)` and `Props1(...)` to the `Props` method instead of calling `_Props1` directly from them.