Fix spelling errors in source comments and output messages
Created by: mtlynch
Fix spelling errors via the misspell tool. Excluded fixes to "marshalled" and
"marshalling" because they seemed to too deeply ingrained in the code.
This is easily repeatable:
go get -u github.com/client9/misspell/cmd/misspell
$GOPATH/bin/misspell -locale US -w -i "marshalled,marshalling,Marshalling,Marshalled" .