feat: rename Exception to ExceptionCode

[CosminPerRam]

In #279 @uklotzde mentioned that:

I have noticed that the naming of the types FunctionCode and Exception is inconsistent. Shouldn't Exception better be renamed to ExceptionCode to reflect this duality?

This PR does exactly this, renames Exception to ExceptionCode.

Also, should associated types be renamed? This is an example:

tokio-modbus/src/server/rtu_over_tcp.rs

Line 224 in a5eb819

type Exception = Exception;

To become:

type ExceptionCode = ExceptionCode

[uklotzde]

Also, should associated types be renamed? This is an example:

tokio-modbus/src/server/rtu_over_tcp.rs

Line 224 in a5eb819

type Exception = Exception;

To become:

type ExceptionCode = ExceptionCode

I don't think so. The type name Exception is appropriate here. It could be any type that could be converted into an ExceptionCode.