catches MultiJson exceptions around keepalive/result queues

catching these exceptions because I'm seeing these messages get
corrupted between the client sending them and the server parsing
them.  if the message is corrupt, it seems best to log it at
error level
guard for the client check requests
user sensu logger's hash context instead of a single long string

addresses comment from @portertech about using the sensu logger's
hash context capacity
use standard error.to_s
missing { added
kind of a dummy change, the build shouldn't be failing
Merge pull request #776 from coffeepac/guard-corrupted-json

Guard corrupted json