bench: surround main() execution with try/catch

so we have a cleaner exit on internal runtime errors.
e.g. an unknown priority level.

src/bench/bench_bitcoin.cpp

@@ -118,6 +118,7 @@ int main(int argc, char** argv)
         return EXIT_SUCCESS;
     }
 
+    try {
         benchmark::Args args;
         args.asymptote = parseAsymptote(argsman.GetArg("-asymptote", ""));
         args.is_list_only = argsman.GetBoolArg("-list", false);
@@ -131,4 +132,8 @@ int main(int argc, char** argv)
         benchmark::BenchRunner::RunAll(args);
 
         return EXIT_SUCCESS;
+    } catch (const std::exception& e) {
+        tfm::format(std::cerr, "Error: %s\n", e.what());
+        return EXIT_FAILURE;
+    }
 }