implement decode exception

src/commonMain/kotlin/com/infendro/encoding/exception/DecodeException.kt

@@ -0,0 +1,5 @@
+package com.infendro.encoding.exception
+
+abstract class DecodeException : Exception() {
+    abstract override val message: String
+}

src/commonMain/kotlin/com/infendro/encoding/exception/IllegalByteException.kt

@@ -0,0 +1,12 @@
+package com.infendro.encoding.exception
+
+class IllegalByteException(
+    val byte: Byte,
+) : DecodeException() {
+    override val message: String
+        get() {
+            val hex = "0x${byte.toHexString().padStart(2, '0')}"
+            val char = "'${byte.toInt().toChar()}'"
+            return "Illegal byte $hex ($char)."
+        }
+}