feat: Begin work on the OmemoManager interface

lib/src/errors.dart

@@ -1,45 +1,46 @@
+abstract class OmemoException {}
+
 /// Triggered during X3DH if the signature if the SPK does verify to the actual SPK.
-class InvalidSignatureException implements Exception {
+class InvalidSignatureException extends OmemoException implements Exception {
   String errMsg() => 'The signature of the SPK does not match the provided signature';
 }
-
 /// Triggered by the Double Ratchet if the computed HMAC does not match the attached HMAC.
 /// Triggered by the Session Manager if the computed HMAC does not match the attached HMAC.
-class InvalidMessageHMACException implements Exception {
+class InvalidMessageHMACException extends OmemoException implements Exception {
   String errMsg() => 'The computed HMAC does not match the provided HMAC';
 }
 
 /// Triggered by the Double Ratchet if skipping messages would cause skipping more than
 /// MAXSKIP messages
-class SkippingTooManyMessagesException implements Exception {
+class SkippingTooManyMessagesException extends OmemoException implements Exception {
   String errMsg() => 'Skipping messages would cause a skip bigger than MAXSKIP';
 }
 
 /// Triggered by the Session Manager if the message key is not encrypted for the device.
-class NotEncryptedForDeviceException implements Exception {
+class NotEncryptedForDeviceException extends OmemoException implements Exception {
   String errMsg() => 'Not encrypted for this device';
 }
 
 /// Triggered by the Session Manager when there is no key for decrypting the message.
-class NoDecryptionKeyException implements Exception {
+class NoDecryptionKeyException extends OmemoException implements Exception {
   String errMsg() => 'No key available for decrypting the message';
 }
 
 /// Triggered by the Session Manager when the identifier of the used Signed Prekey
 /// is neither the current SPK's identifier nor the old one's.
-class UnknownSignedPrekeyException implements Exception {
+class UnknownSignedPrekeyException extends OmemoException implements Exception {
   String errMsg() => 'Unknown Signed Prekey used.';
 }
 
 /// Triggered by the Session Manager when the received Key Exchange message does not meet
 /// the requirement that a key exchange, given that the ratchet already exists, must be
 /// sent after its creation.
-class InvalidKeyExchangeException implements Exception {
+class InvalidKeyExchangeException extends OmemoException implements Exception {
   String errMsg() => 'The key exchange was sent before the last kex finished';
 }
 
 /// Triggered by the Session Manager when a message's sequence number is smaller than we
 /// expect it to be.
-class MessageAlreadyDecryptedException implements Exception {
+class MessageAlreadyDecryptedException extends OmemoException implements Exception {
   String errMsg() => 'The message has already been decrypted';
 }