Review Request: Made /master/log.json endpoint more resilient, and added better logging.

[Benjamin Hindman]

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https://reviews.apache.org/r/5265/
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Review request for mesos and John Sirois.


Summary
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See summary.


Diffs
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  src/master/http.cpp a4cc19d 

Diff: https://reviews.apache.org/r/5265/diff


Testing
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make check


Thanks,

Benjamin