put smp errors into proxy wrappers (#1103)

* put smp errors into proxy wrappers

* use substring in PROXY UNEXPECTED error

* fix encoding

* revert String encoding, discard invalid errors in QC

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Co-authored-by: Evgeny Poberezkin <evgeny@poberezkin.com>

src/Simplex/Messaging/Encoding/String.hs

@@ -75,6 +75,8 @@ instance StrEncoding Str where
   strEncode = unStr
   strP = Str <$> A.takeTill (== ' ') <* optional A.space
 
+-- inherited from ByteString, the parser only allows non-empty strings
+-- only Char8 elements may round-trip as B.pack truncates unicode
 instance StrEncoding String where
   strEncode = strEncode . B.pack
   strP = B.unpack <$> strP