Dedicated errors instead of Strings in messages crate

[bobozaur]

As part of the messages refactor some errors, mostly deserialization ones, were set as String in places like FromStr or TryFrom implementations.

It would be idiomatic to create dedicated types for them along with a crate wide error type. An example of a place that could use a dedicated error type would be the TryFrom<MessageType<'a>> for CredentialPreviewMsgType.

Steps:
- [ ] Move MsgTypeError to the msg_types module.
- [ ] Create crate wide error type.
- [ ] Create dedicated error types instead of String errors.

[SumantxD]

@bobozaur I would like to work on this issue, could you assign it to me plz.