PanicsWithError does not report error message for complex errors

[olivergondza]

When PanicsWithError fails with a message different from the expected one, the message might not indicate very well what the error was, and what the string mismatch is exactly.

It does work in simple cases:

assert.PanicsWithError("expected message", func() {
	panic(errors.New("surprising message"))
})

Fails with:

func (assert.PanicTestFunc)(0x84d480) should panic with error message:	"expected message"
        	       Panic value:	&errors.errorString{s:"surprising message"}
        	       Panic stack:	goroutine 19 [running]:

It does not indicate the actual error message in case the error message is not in the fields:

assert.PanicsWithError("expected message", func() {
	panic(errors.Join(errors.New("surprising message")))
})

Fails with:

func (assert.PanicTestFunc)(0x84d4e0) should panic with error message:	"expected message"
           		Panic value:	&errors.joinError{errs:[]error{(*errors.errorString)(0xc000119570)}}
            		Panic stack:	goroutine 19 [running]:

A real world example here is https://pkg.go.dev/text/template#ExecError.

---

Since the assertion expects this to be an error and it checks on the error message only, it would suffice to replace the listing of entire "Panic value" with panicErr.Error().

[olivergondza]

Proposing fix: #1400