[Crashlytics] Add Error's stacktrace if none were passed

[feinstein]

Currently the lib will get the current stacktrace if none were passed to Crashlytics, but all Error objects have their original stacktrace in them, so we know what was happening when the Error was thrown. This isn't a "one size fits all" solution, but I believe in the majority of cases users will prefer the stacktrace from the Error object than the current stracktrace. One reason I believe in this is because generaly Errors can be thrown from inside a Future and the stacktrace will have a bunch of asynchronous gaps that make the stacktrace useless.

WDYT?