Add details what to do if sourceURLForBridge return nil

[makadaw]

• Add more comments to sourceURLForBridge method
• Move source URL assert on right place in RCTJSExecutor

Thanks for submitting a PR! Please read these instructions carefully:

• Explain the motivation for making this change.
• Provide a test plan demonstrating that the code is solid.
• Match the code formatting of the rest of the codebase.
• Target the master branch, NOT a "stable" branch.

Motivation (required)

RCTBridgeDelegate support 2 ways of JS downloading. First you can return URL (remote or file URL) with - (NSURL *)sourceURLForBridge:(RCTBridge *)bridge method or download it manually with - (void)loadSourceForBridge:(RCTBridge *)bridge onProgress:(RCTSourceLoadProgressBlock)onProgress onComplete:(RCTSourceLoadBlock)loadCallback;.
But if you return nil in sourceURLForBridge and download JS manually RCTJSExecutor will raise assert in - (void)executeApplicationScript:(NSData *)script sourceURL:(NSURL *)sourceURL onComplete:(RCTJavaScriptCompleteBlock)onComplete

With this PR I moved asserts to place where we really need it and updated comments for sourceURLForBridge method

Test Plan (required)

This can be tested with RCTBridgeDelegate that returns nil on sourceURLForBridge and download JS code with - (void)loadSourceForBridge:(RCTBridge *)bridge onProgress:(RCTSourceLoadProgressBlock)onProgress onComplete:(RCTSourceLoadBlock)loadCallback;

[javache]

That's not the right place for the assert, it will only fire now in the case of facebook::react::ScriptTag::RAMBundle:

[makadaw]

Yes, this method use sourceURL only for facebook::react::ScriptTag::RAMBundle

[javache]

sourceURL is also required for executeApplicationScript as otherwise you won't be able to symbolicate your JS.

[makadaw]

Indeed, I see. Ok, I will check in more detail

[javache]

Feel free to open a new PR if you have other changes you want to make here.