Log Message Improvements

[kmaehashi]

Some log messages from Jubatus are hard for operators to understand.

1. Insufficient information
   Some error messages are not self-descriptive.
   For example, just saying "unexpected error" is insufficient. We should clearly state the stituation, like "unexpected exception thrown in mix".
2. Hard to determine the problem
   Similar to 1. The same error messages should not be used more than twice.
   For example, only printing e.what() is done in many location (1, 2, etc). We should give them different prefix.
3. Multiple logs for one situation
   We should print one log for each situation.
   For example, these 3 lines should be unified (or give some common prefix): https://github.com/jubatus/jubatus/blob/b05ad7ffe4a501751645b7e450522bb1e283acc4/jubatus/server/jubavisor/process.cpp#L129-131
4. Format is not unified
   For example, this line print "expected, actual" order, whereas this line prints "actual, expected" order. We should unify them.

[kumagi]

👍
This is serious problem.

[kmaehashi]

Fixed via jubatus/jubatus_core#12 and #822