Expose UnknownBlock error via ApiError

In [certain cases](https://github.com/paritytech/polkadot/issues/5885) a
runtime api is called for an unknown block. For example a block which is
already pruned or on an abandon fork.

In such cases the correct error is returned but it is wrapped in
`ApiError::Application` and the only way to figure out what is the
problem is to inspect the actual message in the error. In polkadot for
example this usually happens when the runtime api version is being
queried. It's beneficial to be able to clearly separate such errors so i
that when they occur the client side can handle them more gracefully.
E.g. log less stressful error message than `State already discarded for
BlockId` or cancel any pending work related on this block.

primitives/api/src/lib.rs

@@ -519,6 +519,8 @@ pub enum ApiError {
 	StateBackendIsNotTrie,
 	#[error(transparent)]
 	Application(#[from] Box<dyn std::error::Error + Send + Sync>),
+	#[error("Api called for an unknownBlock: {0}")]
+	UnknownBlock(String),
 }
 
 /// Extends the runtime api implementation with some common functionality.

primitives/blockchain/src/error.rs

@@ -188,6 +188,7 @@ impl From<Box<dyn sp_state_machine::Error>> for Error {
 impl From<Error> for ApiError {
 	fn from(err: Error) -> ApiError {
 		match err {
+			Error::UnknownBlock(msg) => ApiError::UnknownBlock(msg),
 			Error::RuntimeApiError(err) => err,
 			e => ApiError::Application(Box::new(e)),
 		}