BUG: don't bomb out if we find a package that isn't in npm

[tonywoode]

Describe the bug
I have my own unpublished npm module on my filesystem that I have npm install -g. Making a gist after this has been installed and then trying to do an initial sync fails, and no npm packages at all are installed from the gist:

npm ERR! code E404
npm ERR! 404 Not Found - GET https://registry.npmjs.org/replace-slack-text - Not found
npm ERR! 404
npm ERR! 404  'replace-slack-text@1.1.0' is not in the npm registry.
npm ERR! 404 You should bug the author to publish it (or use the name yourself!)
...

$ npm list -g --depth=0
/Users/twoode/.nvm/versions/node/v12.18.3/lib
├── npm@6.14.6
└── npm-modules-sync@1.2.3

A couple of other reasons users may have packages not in npm: they might have npm linked them, they might use a private repo in addition to npm...

To Reproduce
Steps to reproduce the behavior:

1. make a global package that isn't on npm (eg: npm install -g some trivial thing with a package.json)
2. follow your quickstart instructions to step 3
3. get the above error, note that no packages from the gist have been installed at this point

Expected behavior
If we can't locate a package on npm, we should continue to print the error, but also try to install all other packages

[riyadhalnur]

@tonywoode thank you for filing this! would make for a good feature. I do have plans to add a diff and dry-run flag (#97) for safer syncing. Any PR is always welcome. I don't get much time these days and I'm not sure if the usage stats for this helper warrant any more attention from me for the time being.

[tonywoode]

https://stackoverflow.com/questions/41583514/npm-how-to-sync-global-packages-between-different-computers

it seems to me as if you're the only person in the world making a tool that can replicate an install of the heavily-relied-upon modules from the world's largest code repository. I'm amazed at the lack of usage. People are messy and tend to repeat tasks ;-)

I'm not in any position to do a PR right now i'm afraid, would love to otherwise....