Improve printing of try block expressions

Previously, try blocks didn't quite show their catch statements correctly, and they completely failed to display nicely when a finally block was included:

    julia> :(try; do_try() catch end)
    :(try  # line 1:
            do_try()
        end)

    julia> :(try; do_try() catch do_catch() end)
    :(try  # line 1:
            do_try()
        catch false
            do_catch()
        end)

    julia> :(try; do_try() finally do_final() end)
    :($(Expr(:try, :(begin  # line 1:
            do_try()
        end), false, false, :(begin  # line 1:
            do_final()
        end))))

With this patch, they display correctly and look much more like the written syntax:

    julia> :(try; do_try() catch end)
    :(try  # line 1:
            do_try()
        catch
        end)

    julia> :(try; do_try() catch do_catch() end)
    :(try  # line 1:
            do_try()
        catch
            do_catch()
        end)

    julia> :(try; do_try() finally do_final() end)
    :(try  # line 1:
            do_try()
        finally  # line 1:
            do_final()
        end)
Merge pull request #6857 from mbauman/better-try-expr-show

Improve printing of try block expressions