MLNStyle addSource throws NSException instead of returning NSError

[albertmoravec]

Describe the bug
Adding source via MLNStyle.addSource() twice causes MLNRedundantSourceIdentifierException to be thrown. Since this is an Objective-C exception, AFAIK it is impossible to catch it from Swift without additional tooling in ObjC, ultimately causing my Flutter app to crash.

To Reproduce
Steps to reproduce the behavior:

1. Call MLNStyle.addSource() twice for the same source from Swift
2. NSException is thrown which cannot be handled directly in Swift

Expected behavior
NSError should be returned so it can be properly handled.

Platform information (please complete the following information):

• OS: iOS
• Platform iOS
• Version 16.7.4

[louwers]

Related issue: #1248