Remove action enum, and add Latexify specific exceptions

[odashi]

Overview

Introduces following changes:

• Removes Actions enum and replace them to a raw string.
• Introduces LatexifyError and some subclasses.
• Introduces a behavior when LatexifiedFunction met some LatexifyErrors.

Details

Actions is eventually used as an identical stuff with strings. Maintaining this struct is no longer necessary as some other actions in LatexifyVisitor are defined as raw strings.

LatexifyError indicates internal errors of Latexify itself that can't be recovered by users.
with_latexify now captures this error to prevent destroying the runtime. Jupyter works as follows:

References

NA

Blocked by

NA

[ShigekiKarita]

LGTM