Rewrite error handling in HttpServerConnection#EnsureValidHeaders()

Throwing local exceptions unnecessarily pollutes the exception
stack with immediate unwinding. Avoid this pattern at all cost within
Boost.Coroutines. MSVC may handle exceptions differently and cause
problems with stack unwinding.

refs #7431
refs #7351

lib/remote/httpserverconnection.cpp

@@ -146,43 +146,37 @@ bool EnsureValidHeaders(
 {
 	namespace http = boost::beast::http;
 
-	bool httpError = true;
+	bool httpError = false;
+	String errorMsg;
 
-	try {
-		boost::system::error_code ec;
+	boost::system::error_code ec;
 
-		http::async_read_header(stream, buf, parser, yc[ec]);
+	http::async_read_header(stream, buf, parser, yc[ec]);
 
-		if (ec) {
-			/**
-			 * Unfortunately there's no way to tell an HTTP protocol error
-			 * from an error on a lower layer:
-			 *
-			 * <https://github.com/boostorg/beast/issues/643>
-			 */
-			throw std::invalid_argument(ec.message());
-		}
+	if (ec) {
+		errorMsg = ec.message();
+		httpError = true;
+	}
 
-		httpError = false;
+	switch (parser.get().version()) {
+	case 10:
+	case 11:
+		break;
+	default:
+		errorMsg = "Unsupported HTTP version";
+	}
 
-		switch (parser.get().version()) {
-		case 10:
-		case 11:
-			break;
-		default:
-			throw std::invalid_argument("Unsupported HTTP version");
-		}
-	} catch (const std::invalid_argument& ex) {
+	if (!errorMsg.IsEmpty() || httpError) {
 		response.result(http::status::bad_request);
 
 		if (!httpError && parser.get()[http::field::accept] == "application/json") {
 			HttpUtility::SendJsonBody(response, nullptr, new Dictionary({
 				{ "error", 400 },
-				{ "status", String("Bad Request: ") + ex.what() }
+				{ "status", String("Bad Request: ") + errorMsg }
 			}));
 		} else {
 			response.set(http::field::content_type, "text/html");
-			response.body() = String("<h1>Bad Request</h1><p><pre>") + ex.what() + "</pre></p>";
+			response.body() = String("<h1>Bad Request</h1><p><pre>") + errorMsg + "</pre></p>";
 			response.set(http::field::content_length, response.body().size());
 		}