Replace SdpParserInternalError::Generic with specific errors

[na-g]

If we replace the Generic errors with specific errors, hopefully we can reduce the number of strings that 1) get created, and 2) need localization eventually. We can then test that parsing fails in the expected ways in tests. As it stands those strings are our Error API surface, and changing them could break users of the crate. This should probably be done sooner rather than later.