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