Error handling improvements

[chester-leung]

• Returns client side attestation check failures to Python. Previously, any attestation failure on the client side, e.g. hash mismatch, would print out in C++ but not be returned to Python -- Python would raise a blank XGBoost error. This PR returns a descriptive attestation error to Python, which is then raised in the form of a XGBoost error (with a description of the error).
• Returns error over RPC if thrown on server side
• Wraps up remaining C++ functions called in Python core.py in API_BEGIN() and API_END() calls to ensure smooth error handling. Previously, errors were either printed to console and not returned to Python or were thrown using LOG(FATAL) which would kill the process ungracefully.

[codecov-io]

Codecov Report

Merging #121 into master will decrease coverage by 0.35%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #121      +/-   ##
==========================================
- Coverage   36.39%   36.04%   -0.36%     
==========================================
  Files          16       16              
  Lines        2871     2910      +39     
==========================================
+ Hits         1045     1049       +4     
- Misses       1826     1861      +35     

Impacted Files	Coverage Δ
python-package/securexgboost/remote_server.py	9.45% <0.00%> (-0.02%)	:arrow_down:
python-package/securexgboost/core.py	38.72% <0.00%> (-0.99%)	:arrow_down: