by-AttributeError/KeyError-catchable ObjectMapper attribute misses

[yoloseem]

Motivation:

• KeyError from attribute access is unexpectable.

Changes:

• Raise other exception could be caught by AttributeError, rather than KeyError.
• To prevent unintended breaking on codes may be using KeyError to catch that cases, created MissingFieldError inherited both AttributeError and KeyError.

[coveralls]

Coverage increased (+0.5%) to 89.602% when pulling feb9a90 on yoloseem:attrmiss into 45a9cae on youknowone:master.

[codecov-io]

Current coverage is 89.60%

Merging #27 into master will increase coverage by 0.51%

@@             master        #27   diff @@
==========================================
  Files             8          8          
  Lines           321        327     +6   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits            286        293     +7   
+ Misses           35         34     -1   
  Partials          0          0          

Powered by Codecov. Last updated by 45a9cae...feb9a90

[coveralls]

Coverage increased (+0.2%) to 89.297% when pulling f6a3908 on yoloseem:attrmiss into 45a9cae on youknowone:master.