middleware : catch promise error.

[guillaumearm]

performGetPhrases()(see middleware.js in V2 branch) doesn't catch promise error.

[guillaumearm]

@JalilArfaoui ? what are we doing when getLocale() or getPhrases() reject a value ?

maybe same as #17 ?

[JalilArfaoui]

IMHO, getLocale() returning a falsy value should have the same effect as a non-corresponding action : nothing happens, we simply return next(action). As for getPhrases(), it should always return something. If empty object, we dispatch SET_LANGAGE with empty phrases. If falsy response anyway, we warn about it and return next(action). What do you think ?

[guillaumearm]

yes, ok for this in #17, but what about rejecting value (promise) ? (see PR #43)