Improve JS Exception handling.

- Check for pending exceptions after function calls and script
executions.
- Call LOGERROR instead of JS_ReportError when there is a conversion
error in FromJSVal, since that can only be called from C++ (where JS
errors don't really make sense). Instead, C++ callers of FromJSVal
should handle the failure and, themselves, either report an error or
simply do something else.
- Wrap JS_ReportError since that makes updating it later easier.

This isn't a systematical fix since ToJSVal also ought return a boolean
for failures, and we probably should trigger errors instead of warnings
on 'implicit' conversions, rather a preparation diff.

Part of the SM52 migration, stage: SM45 compatible (actually SM52
incompatible, too).

Based on a patch by: Itms
Comments by: Vladislavbelov, Stan`
Refs #742, #4893

Differential Revision: https://code.wildfiregames.com/D3093
This was SVN commit r24187.

source/scriptinterface/ScriptExceptions.h

@@ -0,0 +1,53 @@
+/* Copyright (C) 2020 Wildfire Games.
+ * This file is part of 0 A.D.
+ *
+ * 0 A.D. is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * 0 A.D. is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with 0 A.D.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef INCLUDED_SCRIPTEXCEPTIONS
+#define INCLUDED_SCRIPTEXCEPTIONS
+
+struct JSContext;
+class JSErrorReport;
+class ScriptRequest;
+
+namespace ScriptException
+{
+/**
+ * @return Whether there is a JS exception pending.
+ */
+bool IsPending(const ScriptRequest& rq);
+
+/**
+ * Log and then clear the current pending exception. This function should always be called after calling a
+ * JS script (or anything that can throw JS errors, such as structured clones),
+ * in case that script doesn't catch an exception thrown during its execution.
+ * If no exception is pending, this does nothing.
+ * Note that JS code that wants to throw errors should throw new Error(...), otherwise the stack cannot be used.
+ * @return Whether there was a pending exception.
+ */
+bool CatchPending(const ScriptRequest& rq);
+
+void ErrorReporter(JSContext* rt, const char* message, JSErrorReport* report);
+
+/**
+ * Raise a JS exception from C++ code.
+ * This is only really relevant in JSNative functions that don't use ObjectOpResult,
+ * as the latter overwrites the pending exception.
+ * Prefer either simply logging an error if you know a stack-trace will be raised elsewhere.
+ */
+void Raise(const ScriptRequest& rq, const char* format, ...);
+}
+
+#endif // INCLUDED_SCRIPTEXCEPTIONS