eliminate errors trying to run a command in a directory that does not exist

[bhcleek]

Fixes #1399

[arp242]

Could you use false, instead of exit? The former is in POSIX, the latter isn't. Also exit is usually a shell built-in, so this code depends on the shell.

You should also use go#util#Exec() for all new code, the System() function will be removed once all calls are rewritten to use that.

[bhcleek]

@Carpetsmoker PTAL

[arp242]

I just added pushed a basic test for this.

I do wonder if silently doing nothing is really the best behaviour though. For example Errcheck will now simply do nothing. I wonder if that's really the best thing to do?

[arp242]

Meh, also seems that Neovim behaves slightly different

[bhcleek]

If go#lint#Errcheck doesn't check the ShellError or is hiding errors for some other reason, then that's a completely separate problem. This change merely makes sure to return early and avoid a bunch of useless, noisy messages when the directory that go#util#ExecuteInDir is supposed to execute in does not exist, but I don't see that behavior. I see a message, "package is not inside GOPATH src"

I'm working on a fix for the nvim behavior. Thanks for adding some tests!

[bhcleek]

PTAL.

I'll rebase before merging to apply the fixups correctly.

[bhcleek]

I'll rebase now and then merge.