Provide better error handling for tight permissions in Artifactory

[briantist]

See also:

• Traceback when publishing amazon.aws #52 (comment)

With only write permissions on a repository, but not update/delete, re-uploading a collection can result in a 404 (if a certain Artifactory setting is enabled), or possibly a 403 otherwise (needs confirmation).

This will cause a stacktrace and an unhelpful response.

We should try to determine when this is the case and return a more helpful error message.

The configuration of being able to upload a collection once but not overwrite it should definitely be supported.

The above would also probably wreak havoc on caching and would likely need caching to be disabled. Possible solutions for that:

• Allow for disabling the upstream proxy cache, and/or caching locally #4 (this possibility adds another vote for a local storage caching option)
• allow for specifying cache responses to go to another artifactory repo entirely, that way the permissions can be separate
  • (check whether artifactory supports permissions at a folder level or only repository level, could be used instead of above)
  • a sub-idea of this: allow an entirely separate repository for all upstream content, including the collections it pulls down
    • this could be nice because it totally separates internal collections from proxied content, which at the moment, are indistinguishable programmatically
    • it would allow for say, more aggressive cleanup of proxied collections
    • would not work for users who populate upstream content separately, unless we have some way to indicate multiple source artifactory repositories
    • implementation could get complicated