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/gui/Scripting/JSInterface_GUISize.cpp

@@ -45,7 +45,7 @@ bool JSI_GUISize::construct(JSContext* cx, uint argc, JS::Value* vp)
 {
 	JS::CallArgs args = JS::CallArgsFromVp(argc, vp);
 	ScriptInterface* pScriptInterface = ScriptInterface::GetScriptInterfaceAndCBData(cx)->pScriptInterface;
-	ScriptInterface::Request rq(*pScriptInterface);
+	ScriptRequest rq(*pScriptInterface);
 
 	JS::RootedObject obj(rq.cx, pScriptInterface->CreateCustomObject("GUISize"));
 
@@ -104,7 +104,7 @@ bool JSI_GUISize::toString(JSContext* cx, uint argc, JS::Value* vp)
 	CStr buffer;
 
 	ScriptInterface* pScriptInterface = ScriptInterface::GetScriptInterfaceAndCBData(cx)->pScriptInterface;
-	ScriptInterface::Request rq(*pScriptInterface);
+	ScriptRequest rq(*pScriptInterface);
 	double val, valr;
 
 #define SIDE(side) \