fix ExitErrHandler not used with subcommands

[travisperson]

Errors returned by subcommands no longer bypass the toplevel ExitErrHandler

Fixes #736

[jkanywhere]

AppVeyer build is failing because go vet

python runtests vet
45# github.com/urfave/cli
46.\command_test.go:328:3: value declared but not used
47vet: typecheck failures
48runtests: go vet ./...
...

which is odd because value defined on line 328 is used on line 340, and this diff doesn't alter that part of the file.

[phinnaeus]

@jkanywhere It's failing on master, without these changes at all. I created #765 to fix that.

Edit: disregard, I'm an idiot. I see you already fixed in #763

[HenrikPoulsen]

This is still an issue. What is preventing us from merging this in at this point?

[coilysiren]

A change identical to this one was merged in this PR => #856