Fix error handling

[hokaccha]

When I have some remote urls, gh-open outputs error message as below:

$ git remote -v
dev     [email protected]:myapp-dev.git (fetch)
dev     [email protected]:myapp-dev.git (push)
heroku  [email protected]:myapp.git (fetch)
heroku  [email protected]:myapp.git (push)
origin  [email protected]:hokaccha/myapp.git (fetch)
origin  [email protected]:hokaccha/myapp.git (push)

$ gh-open .
remote:dev, unsupported remote url: [email protected]:adventar-dev.git
remote:dev, unsupported remote url: [email protected]:adventar-dev.git
remote:heroku, unsupported remote url: [email protected]:adventar.git
remote:heroku, unsupported remote url: [email protected]:adventar.git
# opening github url is successful

This patch fixes the error handling.

# there are github and heroku urls.
$ git remote -v
dev     [email protected]:myapp-dev.git (fetch)
dev     [email protected]:myapp-dev.git (push)
heroku  [email protected]:myapp.git (fetch)
heroku  [email protected]:myapp.git (push)
origin  [email protected]:hokaccha/myapp.git (fetch)
origin  [email protected]:hokaccha/myapp.git (push)

$ gh-open .
# only opening github url

# there is no github url
$ git remote -v
heroku  [email protected]:myapp.git (fetch)
heroku  [email protected]:myapp.git (push)

$ gh-open .
Error: No such GitHub remote url.

# there is no remote url.
$ git remote -v

$ gh-open .
Error: No such GitHub remote url.