feat(cli): add error handling

The CLI now handles error that may result from user input and prints an
error message. Following error types are handled:
 - file not found
 - not a directory
 - directory instead of file
 - file exists
 - permission error
 - general operating system errors
 - Unicode errors
 - errors parsing project source code

Additionally, KeyboardInterrupt and BrokenPipeError are catched and an
appropriate exit code is returned.

src/jcloud_docsgen/cli/cli.py

@@ -22,9 +22,11 @@ __all__ = [
 
 import argparse
 import pathlib
+import sys
 from .parsers.python import VALID_SECTIONS as VALID_PYTHON_SECTIONS
 
 _LANG_PYTHON = 'python'
+_PROGRAM_NAME = 'jcloud-docsgen'
 
 def existing_directory(path: str) -> pathlib.Path:
     _path = path
@@ -35,6 +37,19 @@ def existing_directory(path: str) -> pathlib.Path:
         raise argparse.ArgumentTypeError(f'{_path}: not a directory')
     return path
 
+def error(message: str, exit_code: int = 1):
+    '''
+    Shows an error message and returns the exit code.
+
+    :param message: The error message to show.
+    :type message: str
+    :param exit_code: The exit code.
+    :type exit_code: int
+    '''
+
+    print(f'{_PROGRAM_NAME}: error: {message}', file = sys.stderr)
+    return exit_code
+
 def main(argv = None) -> int:
     common_parser = argparse.ArgumentParser(add_help = False)
     common_parser.add_argument(
@@ -93,15 +108,38 @@ def main(argv = None) -> int:
     if args.output_dir is None:
         args.output_dir = args.project_directory / 'docs/reference'
 
-    if args.language == _LANG_PYTHON:
-        from .parsers.python import generate
-        generate(
-            args.project_directory,
-            args.output_dir,
-            include_sections = args.sections,
-            exclude_sections = args.sections_exclude,
-            allow_html = args.allow_html,
-            allow_tables = args.allow_tables
-        )
+    try:
+        if args.language == _LANG_PYTHON:
+            from .parsers.python import generate
+            generate(
+                args.project_directory,
+                args.output_dir,
+                include_sections = args.sections,
+                exclude_sections = args.sections_exclude,
+                allow_html = args.allow_html,
+                allow_tables = args.allow_tables
+            )
+    except FileNotFoundError as exc:
+        return error(f'{exc.filename}: no such file or directory')
+    except NotADirectoryError as exc:
+        return error(f'{exc.filename}: not a directory')
+    except IsADirectoryError as exc:
+        return error(f'{exc.filename}: is a directory')
+    except FileExistsError as exc:
+        return error(f'{exc.filename}: file already exists')
+    except PermissionError as exc:
+        return error(f'{exc.filename}: permission denied')
+    except OSError as exc:
+        return error(f'{exc.filename}: {exc.args[1]}')
+    except UnicodeError:
+        return error('invalid text encoding')
+    except SyntaxError:
+        # TODO: add a custom exception class to clearify that a project
+        # parsing syntax error is meant.
+        return error(f'syntax error in project source code')
+    except KeyboardInterrupt:
+        return 130
+    except BrokenPipeError:
+        return 141
 
     return 0