Fail flutter build invocation on compilation errors. (#18800)

Fixes https://github.com/flutter/flutter/issues/18200.

packages/flutter_tools/lib/src/commands/build_aot.dart

@@ -93,7 +93,7 @@ class BuildAotCommand extends BuildSubCommand {
           extraFrontEndOptions: argResults[FlutterOptions.kExtraFrontEndOptions],
         );
         if (mainPath == null) {
-          printError('Compiler terminated unexpectedly.');
+          throwToolExit('Compiler terminated unexpectedly.');
           return;
         }
       }