[8.x] Add default parameter to throw_if / throw_unless

[SjorsO]

This PR adds a default parameter to throw_if and throw_unless:

// Currently:
throw_if($somethingIsWrong, new RuntimeException);

// With this PR:
throw_if($somethingIsWrong);

I use throw_if to guard against incorrect situations in non-user facing code. In my case. In most situations I don't care about the specific exception class, or about the exception message. I just want my code to stop running and write a stacktrace to the log file.

[taylorotwell]

Hmm... really seems like you would want a message... I would almost prefer the argument except a string optionally like the @freekmurze updated the report method.