FIX: minor fix on input parsing.

TimeSeries/s2theia.py

@@ -811,7 +811,7 @@ class S2TheiaPipeline:
                             d = datetime.datetime.strptime(align_to, '%Y%m%d')
                             t.rigid_align(ref_date=align_to, match_band=align_using_band-1)
                         except:
-                            warnings.warn("Provided string is not a valid date nor a valid file.")
+                            raise ValueError("Provided string is not a valid date nor a valid file.")
                     elif align_to is None:
                         t.rigid_align(match_band=align_using_band-1)
                 t.reproject(self.output_epsg)