throw if AWS_REGION environment variable is missing (#158)

Author: bmoffatt

tests/runtime_tests.cpp

@@ -44,6 +44,9 @@ struct LambdaRuntimeTest : public ::testing::Test {
         Aws::Client::ClientConfiguration config;
         config.requestTimeoutMs = REQUEST_TIMEOUT;
         config.region = Aws::Environment::GetEnv("AWS_REGION");
+        if (config.region.empty()) {
+            throw std::invalid_argument("environment variable AWS_REGION not set");
+        }
         return config;
     }