Properly disable xfeatures2d if extra files cannot be downloaded

[asmorkalov]

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• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or other license that is incompatible with OpenCV
• The PR is proposed to proper branch
• There is reference to original bug report and related work
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake

[alalek]

Check the issue #1301 and this comment.

Root cause of configuration failure (broken compilation) is that ocv_module_disable() doesn't work properly on the second modules pass (too late).

There are several workarounds:

• update .cache manually
• disable -DBUILD_opencv_xfeatures2d=OFF

Proper fix should look like:

• add "optional" support for these blobs (#ifdef DATA_BLOBS_DOWNLOADED + Error::StsNotImplemented otherwise) instead of whole module disabling
• try to fix ocv_module_disable()

Current proposal just hides the problem (which is not a problem in standard normal build environment). With the same success this can be done through -DBUILD_opencv_xfeatures2d=OFF CMake flag without changes in codebase.

[asmorkalov]

@alalek I added extra definitions and guards in code. The module is always enabled, but some classes can be not implemented (exception).

[alalek]

#2820