capture exceptions thrown by the MongoDB driver
remove debug print
default debug level to 1 (the old behavior)
capture log and detailedLog from backend; print one or the other depending on debugLevel
use scalaz.Either.fromTryCatchNonFatal
feedback on REPL when setting debug level; extend Error
go back to Task.fail for evaluator errors

revert change to use \/ inside the Task
instead, fix the handling of failure in the backend to wrap the
exception with LoggedError
ADT for DebugLevel
clean up diff
make handling of unrecognized DebugLevel code explicit
rename showDebugLevel function
delete commented code
delete commented code
Merge branch 'trap-evaluation-errors' of github.com:mossprescott/slamengine-1 into trap-evaluation-errors
delete file again
Merge pull request #109 from mossprescott/trap-evaluation-errors

capture exceptions thrown by the MongoDB driver
Merge pull request #113 from mossprescott/issue111

#111: log physical plan in BSON form
Merge pull request #114 from mossprescott/issue111

New clean look
Add failing tests for string operations.
Add support for string operations.

`substring`, `lower`, and `upper`. Also added `length`, but it has no implementation on the Mongo side.