Improve error message and README example

[felixge]

• Remove the reference to failing endpoint. It's confusing and redundant (the search and download profiles part of the error message is sufficient to debug which endpoint is being hit)
• Improve the error hint to suggest checking that the account has the right data.
• Improve README example to use double quotes

Before:

ERR search and download profiles: POST /api/unstable/profiles/gopgo: 404 Not Found: please check that your DD_API_KEY, DD_APP_KEY and DD_SITE env vars are set correctly and that your account has profiles matching your query

After:

Jun 13 20:02:05.406 ERR search and download profiles: 404 Not Found: please check that your DD_API_KEY, DD_APP_KEY and DD_SITE env vars are set correctly and that your account has profiles matching your query

See JIRA for more information.

[r1viollet]

LGTM