NCU does not complain about a `configFileName` that does not exist

[matijs]

• [x] I have searched for similar issues
• [x] I am using the latest version of npm-check-updates
• [x] I am using node >= 14.14

---

Steps to Reproduce

.ncurc.cjs:

module.exports = {
  install: 'always',
  upgrade: true,
};

Dependencies:

n/a

Steps:

Current Behavior

# missing the `c` in the config file name...
$ npx ncu --configFileName .ncurc.js
# 🪄 
Run ncu --configFileName .ncurc.js -u to upgrade package.json
$ 

Expected Behavior

It would be cool if ncu could tell me that .ncurc.js did not exist instead of using defaults.

[raineorshine]

Good point. Thanks!

Added a check in 2914dbe.