Do not swallow non-error panics in Encode/Decode

[kolyshkin]

The encoder and decoder report errors by panicking with an error value, which is recovered in (*encoder).Encode and (*decoder).Decode. The recover code, though, silently discards a panic whose value is not an error: Encode returns nil, and Decode returns a partially decoded result with a nil error. Such panics do not come from the encoder/decoder itself (e.g. they may come from the underlying io.Reader or io.Writer), and hiding them is wrong.

Re-panic if the recovered value is not an error, and add tests.

[guelfey]

This is definitely better than before, but thinking about it: the decoder and encoder code explicitly panics at multiple points (using panics more like exceptions for control flow); shouldn't we even use a special error type that gets "detected" within the recover code? Otherwise also other panics that are not from "our" (dbus) code that panic with an err would get swallowed as well. (Or we could just refactor all this to not use panic, but normal error returns...)