Don't panic on unknown error

Currently driver panics on unknown error.
It is better to return generic error with maximum available information
in it.

errors.go

@@ -197,6 +197,10 @@ type RequestErrCASWriteUnknown struct {
 	BlockFor    int
 }
 
+type UnknownServerError struct {
+	errorFrame
+}
+
 type OpType uint8
 
 const (

frame.go

@@ -723,7 +723,9 @@ func (f *framer) parseErrorFrame() frame {
 			res.RejectedByCoordinator = f.readByte() != 0
 			return res
 		} else {
-			panic(fmt.Errorf("unknown error code: 0x%x", errD.code))
+			return &UnknownServerError{
+				errorFrame: errD,
+			}
 		}
 	}
 }