[api] propagate Commissioner construction errors

[wgtdkp]

This PR changes API Commissioner::Create to propagate initialization errors.

[codecov-io]

Codecov Report

Merging #99 into master will decrease coverage by 0.05%. The diff coverage is 77.77%.

@@            Coverage Diff             @@
##           master      #99      +/-   ##
==========================================
- Coverage   60.00%   59.94%   -0.06%     
==========================================
  Files          55       55              
  Lines        5370     5367       -3     
==========================================
- Hits         3222     3217       -5     
- Misses       2148     2150       +2     

Impacted Files	Coverage Δ
include/commissioner/commissioner.hpp	100.00% <ø> (ø)
src/library/commissioner_safe.cpp	63.11% <71.42%> (+0.18%)	:arrow_up:
src/app/commissioner_app.cpp	27.23% <100.00%> (-1.11%)	:arrow_down:
src/app/json.cpp	26.72% <0.00%> (-0.12%)	:arrow_down:
src/common/utils.hpp	65.21% <0.00%> (ø)
src/library/socket.cpp	91.34% <0.00%> (ø)
src/app/commissioner_app.hpp	75.00% <0.00%> (ø)
src/library/network_data.cpp	76.92% <0.00%> (ø)
src/library/commissioner_impl.cpp	61.90% <0.00%> (+0.03%)	:arrow_up:
... and 1 more