Return nice error from handle_prepare if unknown column type in row description

[fishcakez]

It would be good to return a nice error when we don't know how to decode a value, instead of crashing in obscure way after query executed. This means we won't execute a binary query if we don't think we can decode it.

For example (copied from #119 (comment)):

[error] #PID<0.808.0> running MyApp.Endpoint terminated
Server: localhost:4000 (http)
Request: GET /errors/api/errors
** (exit) an exception was raised:
    ** (FunctionClauseError) no function clause matching in Mariaex.Messages.__type__/2
        (mariaex) lib/mariaex/messages.ex:76: Mariaex.Messages.__type__(:type, 245)
        (mariaex) lib/mariaex/row_parser.ex:17: anonymous fn/2 in Mariaex.RowParser.decode_init/1
        (elixir) lib/enum.ex:1755: Enum."-reduce/3-lists^foldl/2-0-"/3
        (mariaex) lib/mariaex/row_parser.ex:16: Mariaex.RowParser.decode_init/1
        (mariaex) lib/mariaex/protocol.ex:537: Mariaex.Protocol.bin_rows_recv/2
        (mariaex) lib/mariaex/protocol.ex:506: Mariaex.Protocol.binary_query_recv/1
        (mariaex) lib/mariaex/protocol.ex:490: Mariaex.Protocol.binary_query_recv/2
        (db_connection) lib/db_connection.ex:957: DBConnection.handle/4
        (db_connection) lib/db_connection.ex:1077: DBConnection.describe_run/5
        (db_connection) lib/db_connection.ex:1141: anonymous fn/4 in DBConnection.run_meter/5
        (db_connection) lib/db_connection.ex:1198: DBConnection.run_begin/3
        (db_connection) lib/db_connection.ex:584: DBConnection.prepare_execute/4
        (ecto) lib/ecto/adapters/sql.ex:243: Ecto.Adapters.SQL.sql_call/6
        (ecto) lib/ecto/adapters/sql.ex:431: Ecto.Adapters.SQL.execute_and_cache/7
        (ecto) lib/ecto/repo/queryable.ex:130: Ecto.Repo.Queryable.execute/5
        (ecto) lib/ecto/repo/queryable.ex:35: Ecto.Repo.Queryable.all/4
        (flames) lib/web/controllers/errors_controller.ex:9: Flames.ErrorsController.index/2
        (flames) lib/web/controllers/errors_controller.ex:2: Flames.ErrorsController.action/2
        (flames) lib/web/controllers/errors_controller.ex:2: Flames.ErrorsController.phoenix_controller_pipeline/2