Report about error during ${push_files} calculation

[ai]

Lefhook: 0.7, 0.6.7, 0.6.3.

Config:

pre-push:
  parallel: true
  commands:
    lint:
      glob: "*.md"
      run: remark --no-stdout --frail --quiet {push_files}

I have 1 unpushed commit with README.md changes.

logux-docs on  master 
➜ g a
## master
M  README.md
M  lefthook.yml

logux-docs on  master [+] 
➜ g c 'Commit'
SYNCING lefthook.yml
SERVED HOOKS: pre-push, prepare-commit-msg
[master 7b4d2ad] Commit
 2 files changed, 3 insertions(+), 9 deletions(-)

logux-docs on  master 
➜ n lefthook run pre-push -v
Lefthook v0.7.0
RUNNING HOOKS GROUP: pre-push

Files before filters: 
 []
Files after filters: 
 []
Files after escaping: 
 []

 lint (SKIP. NO FILES FOR INSPECTING)

SUMMARY: (SKIP EMPTY)

[ai]

Seems like it is DX problem. Lefthook should report about the error during push_files generation:

➜ git diff --name-only HEAD @{push} 
fatal: no upstream configured for branch 'master'