Explicitly return a StateInvalidError from Choice.check.
Revert "Explicitly return a StateInvalidError from Choice.check."

This reverts commit e11e1862e42c5208edd2f4c8940ba8c551747d44.
Throw a StateError when .check() returns something other than a string, LazyText instance or StateInvalidError.
Mention the return types .check() may use to indicate success.
Merge branch 'feature/issue-188-explicit-error-raising-in-choice-check' into develop
Merge branch 'develop' of github.com:praekelt/vumi-jssandbox-toolkit into develop