Add exception types, and a sentinel keyword argument

[gustaphe]

Discussion material for #307
(I may have gone overboard with the error types)

Two concepts to solve the problem of Pluto+Symbolics breaking. Both requires a little intervention at the call site:

1. Use try/catch to guard for LatexifyExceptions, and return something else completely
2. Use the sentinel keyword to make latexify return something

I think I prefer 2, it means everything but the offending type, for instance, gets latexified sensibly. So

julia> struct HasNoRecipe end
julia> latexify([1, "x", HasNoRecipe()]; sentinel="\\mathrm{NA}")
L"\begin{equation}
\left[
\begin{array}{c}
1 \\
x \\
\mathrm{NA} \\
\end{array}
\right]
\end{equation}
"

Not married to any of it, nor the name "sentinel".