Fixed export in Data.Types.Isomorphic .
Rewrote the Exception model.

AskFailure, NetworkError, [NetworkError], and ParamNumError are now
instances of Exception and all exceptions report happens via the
SomeException type.

Other changes:
* The CLI's monad is StateT (ExceptT SomeException IO) a. The ExceptT
layer was added to handle SomeException, since IO only has IOExceptions.
* Functions below the CLI no longer have lists of NetworkException, only
SomeException. However, [e] has been made into an instance of
SomeException, which may be returned.

Smaller changes:
* deleted System.REPL.failureText, System.REPL.Command.maybeToError.
Added state and configuration to SimpleLinearCrawler.

SimpleLinearCrawler m (CrawlerDirection = Forwards/Backwards) (Maybe
Int) is now an instance of StateCrawler and LinearCrawler.

SimpleLinearCrawlers also take their configuration into account;
Forwards results in the same behaviour as previously, Backwards inverts
the direction of crawlerFunction and prevFunc, with crawlerFunction
going backwards and prevFunc going forwards.
Closes #36 and the CLI works again.