Fix #1399: Always report error message on PanicsWithError mismatch

[olivergondza]

Summary

Always report error message on PanicsWithError error message mismatch.

Changes

Add Error message field to the failure message, that reports panicErr.Error() IFF it panicked with an error its message is different from expected.

Motivation

Better defect localization in case of nested errors causing panic.

Before:

Error: func (assert.PanicTestFunc)(0x627a80) should panic with error message:	"at the disco"
	Panic value:	&errors.joinError{errs:[]error{(*errors.errorString)(0xc0000a1540), (*errors.errorString)(0xc0000a1550)}}
	Panic stack:	goroutine 19 [running]:
	...

After:

Error: func (assert.PanicTestFunc)(0x627b20) should panic with error message:	"at the disco"
	Panic value:	&errors.joinError{errs:[]error{(*errors.errorString)(0xc000065550), (*errors.errorString)(0xc000065560)}}
	Error message:	"wrapped err msg\nother err msg"
	Panic stack:	goroutine 6 [running]:
	...

Related issues

Closes #1399