Support try/catch on the happy (nothrow) path

[Pangoraw]

This enables differentation support to try/catch when no exception is thrown on the try block.

The changes are twofolds:

• Stop throwing when encountering :enter and :leave expressions, add an dedicated error message in the adjoint of the catch block instead.
• Make sure that alphas in the first block are stored as stacks if it is a try block since they are not garanteed to run (an exception can be thrown before).

Here is an example of the behavior:

julia> using Zygote

julia> function foo(f)
         y = 1
         try
           y = f()
         catch
           y
         end
         y
       end
foo (generic function with 1 method)

julia> gradient(x -> foo(() -> x), 1) # 1
(1.0,)

julia> _, pull = pullback(foo, () -> 0//0)
(1, Zygote.var"#76#77"{Zygote.Pullback{Tuple{typeof(foo), var"#9#10"}, Any}}(∂(foo)))

julia> pull(1.)
ERROR: Can't differentiate function execution in catch block at #= REPL[4]:4 =#.
Stacktrace:
...

https://github.com/FluxML/IRTools.jl/pull/117 is a companion change that fixes some of the error that can currently happen with try/catch blocks in IRTools.jl.

Supersedes https://github.com/FluxML/Zygote.jl/pull/466.

[ToucheSir]

I'm not really sure how to evaluate any risks from this change other than reading through the old PR. @oxinabox as the reviewer on that, would you be able to take a look here?

[Pangoraw]

Tests are failing because an update to IRTools is needed (https://github.com/FluxML/IRTools.jl/pull/120).

[ToucheSir]

@oxinabox is this still good to merge?

[oxinabox]

I see no reason why it wouldn't be. I thought it was merged back in January