Ensure input to gather_file_paths is a directory

It does not make sense to use this function on a single file

src/sumbuddy/exceptions.py

@@ -3,6 +3,11 @@ def __init__(self, input_directory):
         message = f"The directory {input_directory} and subdirectories (if any) contain no files. \nPlease provide a directory with files."
         super().__init__(message)
 
+class NotADirectoryError(Exception):
+    def __init__(self, input_directory):
+        message = f"The input path '{input_directory}' is not a directory. \nPlease provide a directory with files."
+        super().__init__(message)
+
 class NoFilesAfterFilteringError(Exception):
     def __init__(self, input_directory, ignore_file):
         message = f"The directory {input_directory} contains files, but all are filtered out. \nCheck patterns in your {ignore_file} file and/or hidden files settings."

src/sumbuddy/mapper.py

@@ -1,6 +1,6 @@
 import os
 from sumbuddy.filter import Filter
-from sumbuddy.exceptions import EmptyInputDirectoryError, NoFilesAfterFilteringError
+from sumbuddy.exceptions import EmptyInputDirectoryError, NoFilesAfterFilteringError, NotADirectoryError
 
 class Mapper:
     def __init__(self):
@@ -39,6 +39,10 @@ def gather_file_paths(self, input_directory, ignore_file=None, include_hidden=Fa
         ---------
         file_paths - List. Files in input_directory that are not ignored.
         """
+
+        if not os.path.isdir(input_directory):
+            raise NotADirectoryError(input_directory)
+
         self.reset_filter(ignore_file=ignore_file, include_hidden=include_hidden)
 
         file_paths = []