feat(errors): extract pkg/errors stacktraces

[kattrali]

Goal

Support reporting stack traces captured using the pkg/errors package instead of generating a new stack when Notify() is called.

Closes #128 - updates the premise to use the latest pre-release branch and avoids adding go module files, which will be handled in a dedicated changeset.

Design

Create a new interface for objects implementing StackTrace(), returning an errors.StackTrace instance (which is an array of uintptr representing program counters). Given the array of program counters, the error can be processed normally.

Changeset

• Added errorWithStack to represent the new error type (the errors package does not export this interface but it is listed as stable)

Testing

• Added a new test for wrapping an error using the errors.Wrap() function, which generates a stack trace. This test verifies that the generated stack is used rather than creating a new one when [bugsnag.]errors.New() is called.