Return 400 http error for unknown account

[heifner]

See discussion on #903

Return a 400 http error code for unknown account instead of 500. 400 was chosen instead of 404 because 404 is seen by cleos as a plugin is not enabled. cleos could have been modified, but to better support existing cleos, went with the generic 400 error code.

The following endpoints were updated to return 400 when the account could not be accessed.
/v1/chain/get_abi
/v1/chain/get_code
/v1/chain/get_code_hash
/v1/chain/get_raw_code_and_abi
/v1/chain/get_raw_abi
/v1/chain/get_account
/v1/chain/get_table_rows
/v1/chain/get_currency_balance
/v1/chain/get_currency_stats

./cleos --print-response get account test
RESPONSE:
---------------------
{
  "code": 400,
  "message": "Account lookup",
  "error": {
    "code": 3060002,
    "name": "account_query_exception",
    "what": "Account Query Exception",
    "details": [{
        "message": "unable to retrieve account info (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 test))",
        "file": "chain_plugin.cpp",
        "line_number": 2555,
        "method": "get_account"
      }
    ]
  }
}
---------------------
Error 3060002: Account Query Exception
Most likely, the given account doesn't exist in the blockchain.
Error Details:
unable to retrieve account info (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 test))

Resolves #591

[spoonincode]

s/has/hash/

[spoonincode]

This is the most limited change so far, and it's going in to main. But it still is changing the response code & body for extremely widely used endpoints that have behaved the way they do since 1.0; without performing any sort of versioning of the API. Pretty scary feeling to me. I guess we'll have ample time to communicate the change somehow before 5.0.