Add some exceptions

lib/src/exceptions.dart

@@ -0,0 +1,31 @@
+// Copyright (c) 2020
+// Author: Hugo Pointcheval
+
+class NativeCryptoException implements Exception {
+  String message;
+  NativeCryptoException(this.message);
+}
+
+class KeyException extends NativeCryptoException {
+  KeyException(message) : super(message);
+}
+
+class CipherInitException extends NativeCryptoException {
+  CipherInitException(message) : super(message);
+}
+
+class KemInitException extends NativeCryptoException {
+  KemInitException(message) : super(message);
+}
+
+class EncryptionException extends NativeCryptoException {
+  EncryptionException(message) : super(message);
+}
+
+class DecryptionException extends NativeCryptoException {
+  DecryptionException(message) : super(message);
+}
+
+class NotImplementedException extends NativeCryptoException {
+  NotImplementedException(message) : super(message);
+}