Be more clear with errors which protocol operations can raise them

[richard-jones]

We provide exception names, codes and descriptions, but it's not trivial to match them to which protocol operations raise them, and also in implementation it can be hard to disambiguate them when picking an exception up from the wire.

[richard-jones]

• in 7.1 provide names and links to the error types in 9.8.1
• Protocol requirements should provide name and link to the error type in 9.8.1 when specifying a return type
• create a new major section for errors, and put the error types in there