[lib] Handle string exception in getMessageForException

Summary:
Obvious in hindsight. Only noticed this while debugging an issue where this line was seeing a string exception, which I think is coming from react-native-background-upload.

getMessageForException is meant to be a general-purpose utility, so I think it's good for us to handle this.

Test Plan: I confirmed that after this change, the mediaMissionReport sent by commbot included an exceptionMessage property that had the text of the exception

Reviewers: tomek

Reviewed By: tomek

Subscribers: atul

Differential Revision: https://phab.comm.dev/D5890