Change the interface of CanHandle to return error information.
Fix misunderstanding about when CanHandle fails.
Decapatalise fmt.Errorf error messages.
Change error to warning during handling check.
Squashed commit of the following:

commit 6bf9fe89f638aa1c5f29bd03168311e2097fc8ad
Author: Satnam Singh <satnam@google.com>
Date:   Fri Sep 26 10:23:16 2014 -0700

    Change error to warning during handling check.

commit c58090718309247ae3edd561a7e76d612acbd966
Author: Satnam Singh <satnam@google.com>
Date:   Fri Sep 26 10:21:41 2014 -0700

    Decapatalise fmt.Errorf error messages.

commit 3ecc5745d60b49b0b9b2573684cd27f34364cf3e
Author: Satnam Singh <satnam@google.com>
Date:   Fri Sep 26 10:19:15 2014 -0700

    Fix misunderstanding about when CanHandle fails.

commit adce0c5433946613890c28eec68c8b7798a45633
Author: Satnam Singh <satnam@google.com>
Date:   Fri Sep 26 10:13:32 2014 -0700

    Change the interface of CanHandle to return error information.
Merge branch 'master' of github.com:satnam6502/cadvisor
Merge pull request #256 from satnam6502/master

Change the interface of CanHandle to return error information.