Reset JSContext exception after processing

[nhojb]

The current implementation never resets the JSContext exception property. This means that a single exception will pollute all subsequent calls to MathJaxSwift e.g.

1. try tex2svg(input1) throws
2. MathJax.context.exception is now non-nil
3. try tex2svg(input2) should succeed, but still throws because of input1 exception

The naive fix is to reset the context.exception after checking for the current exception.