panics: allow usage of RecoveredPanic as a normal error

[bobheadxi]

Returning a concrete type *RecoveredPanic that implements error introduces an interesting gotcha - it looks like an error and behaves like an error, but cannot really be used as an error, because if returned as an error even if nil it's a typed error or something and shows up as not nil.

Example using the existing implementation:

func TestCatcherRecovered(t *testing.T) {
	var fn = func() error {
		var c Catcher
		c.Try(func() {})
		return c.Recovered() // ok, looks like an error
	}
	err := fn()
	assert.Nil(t, err)     // ok, surely no error
	assert.NoError(t, err) // fails!! err is nil but is not a nil error
	// Error: Received unexpected error:
	//   	  <nil>
}

Playground example: https://go.dev/play/p/C1DAdBWY9EZ , example in practice: https://github.com/sourcegraph/controller/actions/runs/3972347286/jobs/6810164688#step:7:141

To remedy this, we can either:

• change (*Catcher).Recovered() *RecoveredPanic to (*Catcher).Recovered() error and ask that callers cast it into *RecoveredPanic for details a la os.ExitError
• remove the error implementation and move it to another type that can be retrieved with (*RecoveredPanic).AsError()

This PR currently implements the latter - I might prefer the former, but it's a bigger change, and the distinction between a panic and an error returned is nice.

[codecov-commenter]

Codecov Report

Merging #75 (d3de5bd) into main (e5ed75f) will decrease coverage by 0.26%.
The diff coverage is 90.00%.

❗ Current head d3de5bd differs from pull request most recent head 429a94b. Consider uploading reports for the commit 429a94b to get more accurate results

@@            Coverage Diff             @@
##             main      #75      +/-   ##
==========================================
- Coverage   99.46%   99.21%   -0.26%     
==========================================
  Files          11       11              
  Lines         374      380       +6     
==========================================
+ Hits          372      377       +5     
- Misses          2        3       +1     

Impacted Files	Coverage Δ
panics/panics.go	97.14% <90.00%> (-2.86%)	⬇️

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[camdencheek]

Oof, that's a rough one! Thanks for the fix. I agree that *RecoveredPanic should not implement error. I also agree with your proposed solution. IMO, panics should only rarely be turned into errors, so I think it's fine to have an explicit AsError() method rather than having Recovered() return an error directly.

Just a couple of small comments inline.