Return 500 for non numeric exception codes

[veewee]

Q	A
Type	bug
BC Break	no
Fixed issues	#14

Summary

>>> $exceptionCode = "42S02"
=> "42S02"
>>> $statusCode = $exceptionCode >= 400 && $exceptionCode <= 599 ? $exceptionCode : 500
=> "42S02"

Expected behaviour

An exception passed to Phpro\ApiProblem\Http\ExceptionApiProblem that returns a non-numeric getStatus() (and they do exist) should result in a code of 500, and not throw another exception complaining about the non-numeric status code that it tries to use to pass to the parent constructor (class Phpro\ApiProblem\Http\HttpApiProblem).