Scripting: Add explicit error message when script_score script returns N...

...aN

When a scoring script returns not a number, the current message is confusing (IllegalArgumentException[docID must be >= 0 and < maxDoc=3 (got docID=2147483647)]). This commit adds the error message ScriptException[script score function returns a wrong score: NaN].

Closes #2426
Instead of using an expected exception here, can you use a try/catch in the test and verify the exception messaging? Not exact messaging, just something verifying this is indeed working (since the error before your change was already throwing a SearchPhaseExecutionException).