try-runtime proper error types

[kianenigma]

The current try-runtime code paths are all using a very very rudimentary &'static str as the error type. Even worse, we don't really handle the error in many places because these strings are not all that useful.

Instead, the try-runtime APIs and machinery should either have its very own error type, or even better, use the very flexible DispatchError. All existing &'static str error can be easily converted to DispatchResult via .into().

This is slightly bad in terms of naming, as try-runtime code paths are not necessarily Dispatched, but we really want to reuse DispatchResult's ability to reference exactly which module it is coming from.

If someone works on this and realizes that it is not worthwhile, or if there is a technical issue along the way, we should remove the return type and simply standardize panics in try_state and family.

[Szegoo]

@kianenigma IIUC we should use DispatchError::Other since we don't want to add new variants to the existing enum, right?

[kianenigma]

DispatchError::Other should be used with a &'static str in case no error enum variant from the module can be used .