refactor-for-testability (#3)

Co-authored-by: jdl <jdl@desktop>
Co-authored-by: jdl <jdl@crumpington.com>
Reviewed-on: #3

peer/errors.go

@@ -0,0 +1,10 @@
+package peer
+
+import "errors"
+
+var (
+	errDecryptionFailed  = errors.New("decryption failed")
+	errDuplicateSeqNum   = errors.New("duplicate sequence number")
+	errMalformedPacket   = errors.New("malformed packet")
+	errUnknownPacketType = errors.New("unknown packet type")
+)