Fix get_account fail case, RPC endpoint for is_valid_account

[n13]

Use case:

Any app creating a user account must check if the account exists.

Current solution:
Calling get_account - get_account is relatively expensive when it succeeds but assumes that the account passed in is valid.

get_account does not handle non-existing account well: It crashes with a 500 internal server error, which might be a bug and exploitable by malicious actors.

Fixing get_account is likely also a good idea, so it fails gracefully on a non-existing account

This is a fail case trace:

❯ cleosm --print-request --print-response get account nnnnn       
REQUEST:
---------------------
POST /v1/chain/get_account HTTP/1.1
Host: node.hypha.earth
content-length: 29
Accept: */*
Connection: close

{
  "account_name": "nnnnn"
}
---------------------
RESPONSE:
---------------------
{
  "code": 500,
  "message": "Internal Service Error",
  "error": {
    "code": 0,
    "name": "exception",
    "what": "unspecified",
    "details": [{
        "message": "unknown key (boost::tuples::tuple<bool, eosio::chain::name, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type>): (0 nnnnn)",
        "file": "http_plugin.cpp",
        "line_number": 936,
        "method": "handle_exception"
      }
    ]
  }
}
---------------------
error 2022-12-22T03:39:42.285 thread-0  main.cpp:4371                 operator()           ] Failed with error: unspecified (0)
unknown key (boost::tuples::tuple<bool, eosio::chain::name, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type>): (0 nnnnn)

[heifner]

Related: #188
500 does not indicate a crash, it is just an unfortunate http error code chosen in nodeos for this.

[heifner]

This looks like another request for a is_valid_account RPC endpoint. eosnetworkfoundation/mandel#33

[stephenpdeos]

Alignment that we should verify this is still throwing a 500 error and if so, it should be updated to a 400 or 200 error. We do not plan to add an is_valid endpoint at this time.