memsql errors v0.2.0

Introduces errors.Annotate() and errors.Annotation(), to set an get "annotations" respectively. Annotations are passed as args to capture handlers.

Introduces errors.String.Wrap() and errorString.Unwrap (a bug fix).

What's Changed

• capture_test fixes by @dncohen in #14
• [feat+fix] add string wrap and unwrap by @muir in #15
• [feat] add error annotations by @muir in #16

Full Changelog: v0.1.0...v0.2.0