Created exception utility

[lorenzoPrimi]

Fixes ooni/probe#983

Proposed Changes

• Created a class called ExceptionManager that imports Crashlytics and sends crash to them.
  In this way if we decide to remove Crashlytics and use any other crash reporting system in the code, it will be a one line edit.

[bassosimone]

🐳

Let's also open an issue regarding where to catch exceptions. It may be that we're catching them in too many places and, for sure, I have seen ameliorable code patterns throughout the codebase.