Handle jsonnetfmt errors

[discordianfish]

Hi,

I've been using vim-jsonnet for syntax highlighting successfully for some time but it's not auto-formatting.
Running :JsonnetFmt isn't doing anything / fails silently. jsonnetfmt is in the path and working. Any ideas?

[discordianfish]

Ah! The problem is just that it fails silently when there is a syntax error. Reading the docs it seems like it's not yet supported. Change the title to track this.