Interrupting package install leaves the filesystem in a bad state

[mikrostew]

Found this working on a slow WiFi this afternoon. This is similar to #200, but only for packages.

Interrupting the package install leaves the filesystem in a state where that version of the package is not able to be installed:

$ notion install cowsay 1.4.0
  Unpacking cowsay-v1.4.0  [========================================] 100%
[hit ^C here to interrupt the npm install]

$ notion install cowsay 1.4.0
  Unpacking cowsay-v1.4.0  [========================================] 100%
error: an internal error occurred

Notion is still a pre-alpha project, so we expect to run into some bugs,
but we'd love to hear about them so we can fix them!

Please feel free to reach out to us at @notionjs on Twitter or file an issue at:

    https://github.com/notion-cli/notion/issues

Error details written to: notion-error-2019-04-19_15_28_12.293.log

$ notion install cowsay 1.3.0
  Unpacking cowsay-v1.3.0  [========================================] 100%
npm notice created a lockfile as package-lock.json. You should commit this file.
added 9 packages from 2 contributors and audited 1535 packages in 4.964s
found 0 vulnerabilities