Document the error handling fallback.

core/src/toga/icons.py

@@ -91,7 +91,9 @@ def __init__(
             path, or a path relative to the module that defines your Toga application
             class. This base filename should *not* contain an extension. If an extension
             is specified, it will be ignored. If the icon cannot be found, the default
-            icon will be :attr:`~toga.Icon.DEFAULT_ICON`.
+            icon will be :attr:`~toga.Icon.DEFAULT_ICON`. If an icon file is found, but
+            it cannot be loaded (due to a file format or permission error), an exception
+            will be raised.
         :param system: **For internal use only**
         """
         self.factory = get_platform_factory()

docs/reference/api/resources/icons.rst

@@ -71,9 +71,11 @@ icon variants that are not available from the highest resolution provided (e.g.,
 128px variant can be found, one will be created by scaling the highest resolution
 variant that *is* available).
 
-An icon is **guaranteed** to have an implementation, regardless of the path
-specified. If you specify a path and no matching icon can be found, Toga will
-output a warning to the console, and return :attr:`~toga.Icon.DEFAULT_ICON`.
+An icon is **guaranteed** to have an implementation, regardless of the path specified.
+If you specify a path and no matching icon can be found, Toga will output a warning to
+the console, and return :attr:`~toga.Icon.DEFAULT_ICON`. The only exception to this is
+if an icon file is *found*, but it cannot be loaded (e.g., due to a file format or
+permission error). In this case, an error will be raised.
 
 Reference
 ---------