Make debugger fail less hard

[gnidan]

Might fix some problems in trufflesuite/truffle-debugger#75

• Continue stepping through Solidity code if controller.current.location causes an error
• Use more defaults to prevent unhandled "cannot read whatever of undefined" exceptions

[gnidan]

@seesemichaelj Adding you for review just in case you're interested in getting your eyes on the truffle-debugger. Feel free to disregard.

Thoughts: definitely need to come up with a better strategy for dealing with the debugger when it's in the state of "I don't know where I am!!!" This class of failure case has been entirely disregarded and that needs to be rectified in a more substantial way, instead of these if (!something) return whatever; that just shift the unknown down the line. One idea would involve a conversion to TypeScript, then making use of a Maybe<T> or Option<T> type.

[gnidan]

Note: using the reproduction steps in uport-project/ethr-did-registry#9, I was able to successfully step through the debugger without a single exception (using ; for highest granularity).

[gnidan]

Thanks @cgewecke!

@seesemichaelj I am merging this, just wanted you to have the notification should you be interested.