Remove dependency on error packages

[rhcarvalho]

Summary

At the moment sentry-go has (test) dependency on: github.com/pkg/errors, github.com/go-errors/errors and github.com/pingcap/errors.

Those are not strictly necessary, because all we care about is replicating the particular behavior of those libraries (extended interfaces on their specific error types).

Seldom, but surely, our CI fails unnecessarily because of changes in those dependencies, see
#235 (comment).

In raven-go, this very same removal (for pkg/errors) has been done in getsentry/raven-go#228.

Steps To Reproduce

This is an example of CI failure because of the dependency, but unrelated to the functionality we care about:

• feat: Add Transactions to sentry-go #235 (comment)
  • https://travis-ci.com/github/getsentry/sentry-go/jobs/342625428

Expected Behavior

sentry-go should not depend on the packages listed above. CI should not fail if any of those packages are broken.

Screenshots

SDK

• sentry-go version: master
• Go version: any
• Using Go Modules? [yes/no] no

Additional context

This is also related to #156, as it would reduce the number of entries in go.mod.