DMDecoder: prioritize mirrored format error over normal checksum error

If the symbol is mirrored and has a format error, it could be successfully
error checked and is therefore certainly a mirrored symbol. The old code
hid that fact by instead returning a non-mirrored checksum error.

Found while looking at #774.

core/src/datamatrix/DMDecoder.cpp

@@ -451,7 +451,7 @@ DecoderResult Decode(const BitMatrix& bits)
 	//TODO:
 	// * unify bit mirroring helper code with QRReader?
 	// * rectangular symbols with the a size of 8 x Y are not supported a.t.m.
-	if (auto mirroredRes = DoDecode(FlippedL(bits)); mirroredRes.isValid()) {
+	if (auto mirroredRes = DoDecode(FlippedL(bits)); mirroredRes.error().type() != Error::Checksum) {
 		mirroredRes.setIsMirrored(true);
 		return mirroredRes;
 	}