Go 2 Error Inspection feedback: programmatic Frame inspection

feedback.txt

The proposal provides a means of printing an error, including the source location.
In the proposal, this is the _only_ way of accessing the source location.

In the Elastic APM Go agent (https://github.com/elastic/apm-agent-go), we provide
users with a means of reporting unexpected errors, along with the source location,
in a structured format. As the proposal is, we would need to print and then parse;
and then the format is not well defined anyway.

It would be ideal if we could access the error's frame directly, and then given
a frame, access the source location directly (say, by providing a method to convert
the frame to a runtime.Frame). See feedback_example.go.

feedback_example.go

package errors

type errorString struct {
      s     string
      frame Frame
}

func (e errorString) Frame() Frame {
        return e.frame
}

type Frame struct {
        // Make room for three PCs: the one we were asked for, what it called,
        // and possibly a PC for skipPleaseUseCallersFrames. See:
        // https://go.googlesource.com/go/+/032678e0fb/src/runtime/extern.go#169
        frames [3]uintptr
}

// RuntimeFrame returns f as a runtime.Frame.
func (f Frame) RuntimeFrame() runtime.Frame {
        if _, ok := frames.Next(); !ok {
            return runtime.Frame{}
        }
        fr, ok := frames.Next()
        if !ok {
            return runtime.Frame{}
        }
        return fr
}