Ensure archive directory can be created

Verifies that the archive directory can be created as soon as the
document is set to be archived.

dangerzone/document.py

@@ -77,6 +77,11 @@ class Document:
             # in unwriteable directory
             raise errors.UnwriteableOutputDirException()
 
+    def validate_default_archive_dir(self) -> None:
+        """Checks if archive dir can be created"""
+        if not os.access(self.default_archive_dir.parent, os.W_OK):
+            raise errors.UnwriteableArchiveDirException()
+
     @property
     def input_filename(self) -> str:
         if self._input_filename is None:
@@ -125,6 +130,7 @@ class Document:
     @archive_after_conversion.setter
     def archive_after_conversion(self, enabled: bool) -> None:
         if enabled:
+            self.validate_default_archive_dir()
             self._archive = True
         else:
             self._archive = False

dangerzone/errors.py

@@ -71,6 +71,15 @@ class OutputDirIsNotDirException(DocumentFilenameException):
         super().__init__("Specified output directory is actually not a directory")
 
 
+class UnwriteableArchiveDirException(DocumentFilenameException):
+    """Exception for when the archive directory cannot be created."""
+
+    def __init__(self) -> None:
+        super().__init__(
+            "Archive directory for storing unsafe documents cannot be created."
+        )
+
+
 class SuffixNotApplicableException(DocumentFilenameException):
     """Exception for when the suffix cannot be applied to the output filename."""