wcst_import - fix error with variable not defined

[Bang Pham Huu]

when "skip": true and an input file is not accessible, then this error appears.

WARNING: input file '/test/Sentinel-2/MSI/L2A/2017/01/02/S2A_MSIL2A_20170102T093402_N0204_R136_T34UGU_20170102T093400.SAFE/GRANULE/L2A_T34UGU_A007998_20170102T093400/IMG_DATA/R10m/L2A_T34UGU_20170102T093402_B04_10m.jp2' cannot be processed,
wcst_import will ignore this file as "skip" is set to true in the ingredient file. Reason: `/test/Sentinel-2/MSI/L2A/2017/01/02/S2A_MSIL2A_20170102T093402_N0204_R136_T34UGU_20170102T093400.SAFE/GRANULE/L2A_T34UGU_A007998_20170102T093400/IMG_DATA/R10m/L2A_T34UGU_20170102T093402_B04_10m.jp2' not recognized as a supported file format.
Elapsed time: 0.538 s.
An error has occured in the execution of the program. Error Message: local variable 'first_coverage_slice' referenced before assignment
Stack Trace: Traceback (most recent call last):
  File "/opt/rasdaman/share/rasdaman/wcst_import/wcst_import.py", line 201, in main
    reg.run_recipe(session)
  File "/opt/rasdaman/share/rasdaman/wcst_import/master/recipe/recipe_registry.py", line 272, in run_recipe
    self.__run_recipe(session, recipe)
  File "/opt/rasdaman/share/rasdaman/wcst_import/master/recipe/recipe_registry.py", line 218, in __run_recipe
    recipe.describe()
  File "/opt/rasdaman/share/rasdaman/wcst_import/recipes/general_coverage/recipe.py", line 151, in describe
    importer = self._get_importer()
  File "/opt/rasdaman/share/rasdaman/wcst_import/recipes/general_coverage/recipe.py", line 810, in _get_importer
    coverages = self._get_coverages()
  File "/opt/rasdaman/share/rasdaman/wcst_import/recipes/general_coverage/recipe.py", line 671, in _get_coverages
    coverages = self._get_gdal_coverages(recipe_type)
  File "/opt/rasdaman/share/rasdaman/wcst_import/recipes/general_coverage/recipe.py", line 695, in _get_gdal_coverages
    coverages = GdalToCoverageConverter(self.resumer, self.session.default_null_values,
  File "/opt/rasdaman/share/rasdaman/wcst_import/recipes/general_coverage/abstract_to_coverage_converter.py", line 541, in to_coverages
    global_metadata = self._generate_global_metadata(first_coverage_slice)
UnboundLocalError: local variable 'first_coverage_slice' referenced before assignment