skip is true but wcst_import stopped on missing file

[Dimitar Misev]

The ingredients file has "skip": true,, but because of a missing file wcst_import stopped the import:

An error has occured in the execution of the program. Error Message: `/vsis3/s2-l2a-2018-q3/29/R/MQ/2018/08/30/S2B_MSIL2A_20180830T110609_N0208_R137_T29RMQ_20180830T171510/GRANULE/L2A_T29RMQ_A007742_20180830T111449/IMG_DATA/R20m/T29RMQ_20180830T110609_B03_20m.jp2' does not exist in the file system, and is not recognized as a supported dataset name.
Stack Trace: Traceback (most recent call last):
  File "/opt/rasdaman/share/rasdaman/wcst_import/wcst_import.py", line 192, in main
    reg.run_recipe(session)
  File "/opt/rasdaman/share/rasdaman/wcst_import/master/recipe/recipe_registry.py", line 222, in run_recipe
    self.__run_recipe(session, recipe)
  File "/opt/rasdaman/share/rasdaman/wcst_import/master/recipe/recipe_registry.py", line 168, in __run_recipe
    recipe.describe()
  File "/opt/rasdaman/share/rasdaman/wcst_import/recipes/general_coverage/recipe.py", line 149, in describe
    importer = self._get_importer()
  File "/opt/rasdaman/share/rasdaman/wcst_import/recipes/general_coverage/recipe.py", line 719, in _get_importer
    self.importer = Importer(self.resumer, self._get_coverage(), self.options['wms_import'], self.options['scale_levels'],
  File "/opt/rasdaman/share/rasdaman/wcst_import/recipes/general_coverage/recipe.py", line 629, in _get_coverage
    coverage = self._get_gdal_coverage(recipe_type)
  File "/opt/rasdaman/share/rasdaman/wcst_import/recipes/general_coverage/recipe.py", line 651, in _get_gdal_coverage
    self.options['tiling'], self._global_metadata_fields(),
  File "/opt/rasdaman/share/rasdaman/wcst_import/recipes/general_coverage/recipe.py", line 413, in _global_metadata_fields
    metadata_dict = GdalToCoverageConverter.parse_gdal_global_metadata(file_path)
  File "/opt/rasdaman/share/rasdaman/wcst_import/recipes/general_coverage/gdal_to_coverage_converter.py", line 104, in parse_gdal_global_metadata
    gdal_dataset = GDALGmlUtil(file_path)
  File "/opt/rasdaman/share/rasdaman/wcst_import/util/gdal_util.py", line 46, in __init__
    self.gdal_dataset = gdal.Open(str(self.gdal_file_path).encode('utf8'))
  File "/usr/lib/python3/dist-packages/osgeo/gdal.py", line 3010, in Open
    return _gdal.Open(*args)
RuntimeError: `/vsis3/s2-l2a-2018-q3/29/R/MQ/2018/08/30/S2B_MSIL2A_20180830T110609_N0208_R137_T29RMQ_20180830T171510/GRANULE/L2A_T29RMQ_A007742_20180830T111449/IMG_DATA/R20m/T29RMQ_20180830T110609_B03_20m.jp2' does not exist in the file system, and is not recognized as a supported dataset name.