Improvement to Halo2 Prepare phase

[ElusAegis]

1. Fixed an issue with hardcoded names of circuit prepare files (keys and SRS), now is adjusted based on the circuit name.
2. Improved error reporting and suggesting to the user what the issue might be.
3. Added an option to manually add circuit files (if they exist, the mopro prepare will avoid regenerating them.
4. Added an explicit description of the purpose of the binary in the example/halo2/fibonacci