Display expressions with Exception output

[wwitzel]

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It would be very useful to see the LaTeX form of expressions when they are referenced in exception messages, where possible. Sometimes the LaTeX cannot be generated or compiled, in which case the string form is a reasonable fallback. But otherwise, the png would be valuable as well as a link to the expression information page. Furthermore, in messages that indicate that two expressions are not equal to each other but should be, we should link to some kind of "expression diff" page. "Expression diff" would really be feature in its own right, but I'll just lump it in with this issue.

One way the "diff" could work is to put the expression info of the two expressions in the same table side-by-side. Wherever their LaTeX and Expression-type is the same, it can show it as merged as if it was for a single expression. Where they differ, it splits it in two and maybe marks it in red (or some such thing to highlight the difference region). There may be better ways, though.