fix: change error to warning when canceling npm login or adduser

[mbtools]

Problem

Canceling npm login or npm adduser leads to an error message:

Solution

npm now issues a simple warning (same as for npm init):

[wraithgar]

Is it appropriate that the exit code of npm is now 0 instead of 1 on this use case?

[mbtools]

Good for user, no good for automation. 👎