Add status code check for http file sources

[praveenrewar]

What this PR does / why we need it:

• Use http.Client instead of http to make get calls
• Add status code check to error out for non OK http status codes
  (OK = 2xx)
• Add unit tests for http file sources

Which issue(s) this PR fixes:

Fixes #521

Does this PR introduce a user-facing change?

NONE

Additional Notes for your reviewer:

Review Checklist:

• Follows the developer guidelines
• Relevant tests are added or updated
• Relevant docs in this repo added or updated
• Relevant carvel.dev docs added or updated in a separate PR and there's
  a link to that PR
• Code is at least as readable and maintainable as it was before this
  change

Additional documentation e.g., Proposal, usage docs, etc.:

Current behaviour

$ kapp deploy -a test -f https://github.com/fake/path/
Target cluster 'https://192.168.64.93:8443' (nodes: minikube)

kapp: Error: error converting YAML to JSON: yaml: line 163: mapping values are not allowed in this context

Expected behaviour:

$ /kapp deploy -a test -f https://github.com/fake/path/                                      
Target cluster 'https://192.168.64.93:8443' (nodes: minikube)

kapp: Error: Requesting URL 'https://github.com/fake/path/': 404 Not Found

[100mik]

LGTM!
I think using status OK would be more appropriate for the test, but since we are going ahead with a range, I guess it makes sense to have something that is in the middle 🚀

[kumaritanushree]

LGTM