Remove panics in 'to protobuf' logic of the converter

[dc7303]

What this PR does / why we need it:
If an error occurs in the converter, the converter must return an error
and allow the caller to decide how to handle it.

Which issue(s) this PR fixes:

Fixes #50

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Additional documentation:

Checklist:

• Added relevant tests or not required
• Didn't break anything

[codecov]

Codecov Report

Merging #135 (ec02d1d) into master (d68e7cb) will decrease coverage by 0.28%.
The diff coverage is 73.71%.

@@            Coverage Diff             @@
##           master     #135      +/-   ##
==========================================
- Coverage   62.25%   61.96%   -0.29%     
==========================================
  Files          26       26              
  Lines        2673     2758      +85     
==========================================
+ Hits         1664     1709      +45     
- Misses        860      882      +22     
- Partials      149      167      +18     

Impacted Files	Coverage Δ
yorkie/rpc/server.go	49.59% <37.50%> (-1.28%)	⬇️
api/converter/to_bytes.go	85.15% <57.89%> (-11.00%)	⬇️
api/converter/to_pb.go	82.84% <82.14%> (-4.00%)	⬇️

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[hackerwins]

Thank you for your work. 👍 I left two types of comments.

• Whether fmt.Errorf is properly formatted.
• Apply pbXXX name to protobuf message.

[dc7303]

@hackerwins
Thank you for the meticulous review! Corrected the issues you suggested :)

[hackerwins]

LGTM. 👍 Thanks for contributing.