cover: Use getInputStream() inline.

It's readable, and will make try-with-resources conversion really easy when
that happens.
cover: Initialize the result with null, don't return early.

Avoid early returns when it makes sense, makes things easier to read and less
bug prone.
AbstractWebCover: Try to close the BufferedReader in final().
AbstractWebCover: Simplify while loop for reading.
CoverManager: Properly close resources after use.
AbstractWebCover: Make sure httpPost/httpGet exist before isAborted().
CoverManager: Make sure conMan exists before attempting use.

It's probably paranoid, but it takes no resources to check and ensure that
conMan exists before use.
AbstractWebCover: Move client to the calling methods, close them in final().