Better error if plugin file in baseline not found

Fixes: #718

detect_secrets/core/plugins/initialize.py

@@ -31,7 +31,17 @@ def from_plugin_classname(classname: str) -> Plugin:
     """
     :raises: TypeError
     """
-    for plugin_type in get_mapping_from_secret_type_to_class().values():
+    try:
+        plugin_types = get_mapping_from_secret_type_to_class().values()
+    except FileNotFoundError as e:
+        log.error(f'Error: Failed to load `{classname}` plugin: {e}')
+        log.error(
+            'This error can occur when using a baseline that references a '
+            'custom plugin with a path that does not exist.',
+        )
+        raise
+
+    for plugin_type in plugin_types:
         if plugin_type.__name__ == classname:
             break
     else:

detect_secrets/core/scan.py

@@ -138,8 +138,12 @@ def scan_line(line: str) -> Generator[PotentialSecret, None, None]:
 
 
 def scan_file(filename: str) -> Generator[PotentialSecret, None, None]:
-    if not get_plugins():   # pragma: no cover
-        log.error('No plugins to scan with!')
+    try:
+        if not get_plugins():   # pragma: no cover
+            log.error('No plugins to scan with!')
+            return
+    except FileNotFoundError as e:
+        log.error('Unable to load plugins!')
         return
 
     if _is_filtered_out(required_filter_parameters=['filename'], filename=filename):

detect_secrets/util/importlib.py

@@ -1,3 +1,4 @@
+import errno
 import importlib.util
 import os
 import pkgutil
@@ -85,7 +86,8 @@ def import_file_as_module(filename: str, name: Optional[str] = None) -> ModuleTy
     for you.
     """
     if not os.path.exists(filename):
-        raise FileNotFoundError
+        # Source: https://stackoverflow.com/a/36077407
+        raise FileNotFoundError(errno.ENOENT, os.strerror(errno.ENOENT), filename)
 
     if not name:
         # NOTE: After several trial and error attempts, I could not discern the importance