Implements the Message related changes of #1207.

The idea here is that every message now returns a Result<Message::Output, LangError>,
where a LangError is a type of error which doesn't originate from the contract itself,
nor from the underlying execution environment (so the Contracts pallet in this case).

An example of where this would arise is if a caller tries to use a non-existent message
selector for a contract. Previously, the contract would trap and not allow the caller to
do any sort of error handling if it encountered a non-existent selector.

This breaks the ABI in two ways: first, all contract messages now have a Result return
type, and second a new field, lang_err, will be introduced as part of the contract
spec. The second change allows other languages, such as Solang, to use an equivalent
LangError.

"messages": [
  {
    "args": [],
    "docs": [
      " Flips the current value of the Flipper's boolean."
    ],
    "label": "flip",
    "mutates": true,
    "payable": false,
    "returnType": {
      "displayName": [
        "ink",
        "MessageResult"
      ],
      "type": 1
    },
    "selector": "0x633aa551"
  }],
"lang_error": {
  "displayName": [
    "ink",
    "LangError"
  ],
  "type": 3
},
{
  "id": 3,
  "type": {
    "def": {
      "variant": {
        "variants": [
          {
            "index": 1,
            "name": "CouldNotReadInput"
          }
        ]
      }
    },
    "path": [
      "ink_primitives",
      "LangError"
    ]
  }
}

TODO:

• Add new LangError type to metadata
• Add E2E test for demonstrating how to handle LangErrors
• Make CI green

TODOs in Follow-Ups:

• Implement changes for constructors (Handle LangError from instantiate #1512)
• Implement changes for traits (Fix trait message return type metadata #1531)
• Refactor dispatch logic to bubble up specific dispatch error
  • Will happen at the expense of always having an extra error variant in LangError
• Make the CallBuilder API not require an explicit Result (Clean up CallBuilder return() type #1525)

cc @xgreenx since you were the original poster for the issue