Add a preconditions module, and refactor errors

100% code coverage for the preconditions module.

Usage:
```
$.checkState(something === anotherthing, 'Expected something to be
anotherthing');
$.checkArgument(something < 100, 'something', 'must be less than 100');
$.checkArgumentType(something, PrivateKey, 'something'); // The third
argument is a helper to mention the name of the argument
$.checkArgumentType(something, PrivateKey); // but it's optional (will
show up as "(unknown argument)")
```
Merge pull request #695 from eordano/feature/errors

Add a preconditions module, and refactor errors