Fix ImportError and SyntaxError handling

* Fixes ImportError handling for BlueprintAutoDiscoveryRegistry.

* Adds proper handling of SyntaxError during module discovery.

* Fixes silencing of both ImportError and SyntaxError.

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
Allow removal of registries

* Changes behavior to allow removal of a registry after it has
  been added to application registry.

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>