error handling

[wenxi-zeng]

• better error reporting mechanism. now Analytics reports a series of errors that user can handle accordingly to their needs
• added 3 new APIs
  • reportInternalError to direct exceptions to user provided error handler
  • pendingUploads to get the yet uploaded files
  • purgeStorage to remove all local files
  • purgeStorage(filePath: String) to remove a specific file
• deprecated the use of ICoroutineExceptionHandler. promote to use IAnalyticsErrorHandler instead