Obtain DispatchError::Module info dynamically (#453)

* Add error information back into metadata to roll back removal in #394

* Go back to obtaining runtime error info

* re-do codegen too to check that it's all gravy

* Convert DispatchError module errors into a module variant to make them easier to work with

* Fix broken doc link

subxt/src/lib.rs

@@ -81,6 +81,7 @@ pub use crate::{
         BasicError,
         Error,
         GenericError,
+        HasModuleError,
         RuntimeError,
         TransactionError,
     },
@@ -98,6 +99,7 @@ pub use crate::{
         UncheckedExtrinsic,
     },
     metadata::{
+        ErrorMetadata,
         Metadata,
         MetadataError,
         PalletMetadata,