Don't fail on pnpm install without a package.json

[rstacruz]

Doing pnpm install in a path without package.json shows a cryptic error message and exits with code 1.

~ $ cd /tmp
/tmp $ pnpm i

 ! Cannot read property 'dependencies' of undefined

For comparison, npm is also a little strange in its behavior in this case, but it at least exits successfully:

/tmp $ npm i
npm WARN enoent ENOENT: no such file or directory, open '/private/tmp/package.json'
npm WARN tmp No description
npm WARN tmp No repository field.
npm WARN tmp No README data
npm WARN tmp No license field.

# node_modules will be enumerated here, if it exists

Related #47.

[zkochan]

Currently pnpm does have a meaningful error message in this situation. It says: ERROR No package.json found

[jsumners]

I routinely do an npm install foo in an empty directory to try out a package. This is functionality I expect to work in any package manager.