Throws its own exceptions

[maidmaid]

I'm writting a validator based on exceptions thrown by LocalDateInterval but we need at the moment to catch Webmozart exceptions to handle domain errors.

try {
    LocalDateInterval::parse($text);
} catch (\Gammadia\DateTimeExtra\Exceptions\IntervalParseException $e) {
    // handles parsing errors
} catch (\Webmozart\Assert\InvalidArgumentException $e) {
    // handles domain errors
}

This PR throws its own exceptions when it's domain specific and continues to call Webmozart namespaced exception when a error has to be handle in development process.

catch (\Gammadia\DateTimeExtra\Exceptions\IntervalParseException $e) { // handles parsing errors }
catch (\Gammadia\DateTimeExtra\Exceptions\InvalidArgumentException $e) { // handles domain errors }

A base exception can also be caught for generic purpose :

catch (\Gammadia\DateTimeExtra\Exceptions\ExceptionInterface $e) {}

[maidmaid]

cf webmozarts/assert#17