Fix exception handling in TProcessor

[microdog]

This fixes the issue that exceptions may be dropped silently in TProcessor. The issue was introduced in #4.

[codecov]

Codecov Report

Merging #128 into master will increase coverage by 3.39%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #128      +/-   ##
==========================================
+ Coverage   79.71%   83.11%   +3.39%     
==========================================
  Files          43       43              
  Lines        3905     3908       +3     
==========================================
+ Hits         3113     3248     +135     
+ Misses        792      660     -132

Impacted Files	Coverage Δ
thriftpy2/contrib/aio/processor.py	84.31% <100%> (+2.31%)	⬆️
thriftpy2/contrib/tracking/__init__.py	96.45% <100%> (+35.73%)	⬆️
thriftpy2/tornado.py	85.18% <100%> (+2.57%)	⬆️
thriftpy2/contrib/aio/socket.py	78.02% <0%> (+0.54%)	⬆️
thriftpy2/contrib/tracking/tracker.py	86.73% <0%> (+8.16%)	⬆️
thriftpy2/http.py	87.35% <0%> (+8.62%)	⬆️
thriftpy2/thrift.py	88.88% <0%> (+11.82%)	⬆️
thriftpy2/server.py	64.86% <0%> (+27.02%)	⬆️

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[ethe]

LGTM