cover: Abstract out HTTP status code checking.

Also, AS took this opportunity to optimize imports.
AbstractWebCover: Move the URL build out of executeGetRequestWithConnection().

Obviously, AS took this opportunity to optimize the imports of CoverManager
as well.
cover: Move HTTP connection building to it's own method.
cover: Check the status code directly after requesting it.
cover: Add a new method to check url status by connection, rather than status.
cover: Use getInputStream() inline so we don't need to close it.

According to a StackOverflow user, Java closes all children resources passed from
arguments, so utilize that.
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.