Replace pkg/errors with Go standard library error handling

[stickpro]

Currently the project uses github.com/pkg/errors for error wrapping and stack traces.
Since Go 1.13, the standard library provides built-in error wrapping and inspection via:
errors.Is
errors.As
errors.Unwrap
fmt.Errorf("... %w", err)
This makes pkg/errors redundant for most use cases and allows us to reduce external dependencies.