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/network/NetClient.h

@@ -156,7 +156,7 @@ public:
 	template<typename... Args>
 	void PushGuiMessage(Args const&... args)
 	{
-		ScriptInterface::Request rq(GetScriptInterface());
+		ScriptRequest rq(GetScriptInterface());
 
 		JS::RootedValue message(rq.cx);
 		ScriptInterface::CreateObject(rq, &message, args...);