Fix file not found error for get_preview script

[codemonkey800]

I noticed there was an issue with the recent PR to napari-zelda resulting in the preview page throwing an error on build: https://github.com/RoccoDAnt/napari-zelda/runs/4719127633?check_suite_focus=true

Traceback (most recent call last):
  File "napari-hub/backend/get_preview.py", line 17, in <module>
    get_plugin_preview(repo_pth, dest_pth, args.local, args.branch)
  File "/home/runner/work/napari-zelda/napari-zelda/napari-hub/backend/preview/preview.py", line 48, in get_plugin_preview
    github_metadata = get_github_metadata(action_repo_url, branch=branch)
  File "/home/runner/work/napari-zelda/napari-zelda/napari-hub/backend/utils/github.py", line 123, in get_github_metadata
    citation_file = get_file(repo_url, "CITATION.cff", branch=branch)
  File "/home/runner/work/napari-zelda/napari-zelda/napari-hub/backend/utils/github.py", line 51, in get_file
    with open(os.path.join(local_workspace, file)) as f:
FileNotFoundError: [Errno 2] No such file or directory: '/home/runner/work/napari-zelda/napari-zelda/CITATION.cff'

It looks like this is because there isn't error handling for when a file doesn't exist in a local workspace 🤔

napari-hub/backend/utils/github.py

Lines 49 to 52 in e2ee13d

	if local_workspace:
	# read files locally since github action already checked it out
	with open(os.path.join(local_workspace, file)) as f:
	return f.read()

Adding exception handling for FileNotFoundError should fix the issue 😄