use dedicated exception for loading errors

fixes #145

nvchecker/__main__.py

@@ -33,8 +33,8 @@ def main() -> None:
   try:
     entries, options = core.load_file(
       args.file, use_keymanager=not bool(args.keyfile))
-  except FileNotFoundError:
+  except core.FileLoadError as e:
-    sys.exit('version configuration file not given and default does not exist')
+    sys.exit(str(e))
 
   if args.keyfile:
     keymanager = KeyManager(Path(args.keyfile))

nvchecker/core.py

@@ -140,11 +140,23 @@ class Options(NamedTuple):
   proxy: Optional[str]
   keymanager: KeyManager
 
+class FileLoadError(Exception):
+  def __init__(self, kind, exc):
+    self.kind = kind
+    self.exc = exc
+
+  def __str__(self):
+    return f'failed to load {self.kind}: {self.exc}'
+
 def load_file(
   file: str, *,
   use_keymanager: bool,
 ) -> Tuple[Entries, Options]:
-  config = toml.load(file)
+  try:
+    config = toml.load(file)
+  except OSError as e:
+    raise FileLoadError('version configuration file', e)
+
   ver_files: Optional[Tuple[Path, Path]] = None
   keymanager = KeyManager(None)
 
@@ -167,7 +179,10 @@ def load_file(
         keyfile_s = os.path.expandvars(
           os.path.expanduser(c.get('keyfile')))
         keyfile = d / keyfile_s
-      keymanager = KeyManager(keyfile)
+      try:
+        keymanager = KeyManager(keyfile)
+      except OSError as e:
+        raise FileLoadError('keyfile', e)
 
     max_concurrency = c.get('max_concurrency', 20)
     proxy = c.get('proxy')