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/test_setup.cpp

@@ -153,6 +153,5 @@ void ScriptTestSetup(const ScriptInterface& scriptinterface)
 	std::ifstream ifs(OsString(path).c_str());
 	ENSURE(ifs.good());
 	std::string content((std::istreambuf_iterator<char>(ifs)), std::istreambuf_iterator<char>());
-	bool ok = scriptinterface.LoadScript(L"test_setup.js", content);
-	ENSURE(ok);
+	ENSURE(scriptinterface.LoadScript(L"test_setup.js", content));
 }