Catch Sleigh exceptions

[kkaempf]

This patch is adding some exception handling to prevent sleigh from dumping core.

This is especially useful when developing a new processor implementation (with possible specification errors).

[CLAassistant]

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[ekilmer]

Hello @kkaempf, thank you again for your contribution and improvement! I am happy to merge this in, but I have to ask again if you would sign the CLA (sorry, company policy)