alternator: add "LimitExceededException" error type

This patch adds to Alternator's api_error type yet another type of
error, api_error::limit_exceeded (error code "LimitExceededException").
DynamoDB returns this error code in certain situations where certain
low limits were exceeded, such as the case we'll need in a following
patch - an UpdateTable that tries to create more than one GSI at once.

The LimitExceededException error type should not be confused with
other similarly-named but different error messages like
ProvisionedThroughputExceededException or RequestLimitExceeded.
In general, we make an attempt to return the same error code that
DynamoDB returns for a given error.

Signed-off-by: Nadav Har'El <nyh@scylladb.com>

alternator/error.hh

@@ -88,6 +88,9 @@ public:
     static api_error table_not_found(std::string msg) {
         return api_error("TableNotFoundException", std::move(msg));
     }
+    static api_error limit_exceeded(std::string msg) {
+        return api_error("LimitExceededException", std::move(msg));
+    }
     static api_error internal(std::string msg) {
         return api_error("InternalServerError", std::move(msg), http::reply::status_type::internal_server_error);
     }