Narrow bare except blocks so real errors aren't swallowed (roadmap A4)

Replaced all 46 bare 'except:' across the package with specific exception
types, and logged the cases that indicate a real problem:

- config load/save: FileNotFoundError is the silent first-run case; other
  OSError/ValueError/UnicodeDecodeError are logged.
- ffprobe/avprobe: JSON parse failures are now logged (previously a media
  file that ffprobe couldn't read just silently failed to import); decode
  fallbacks -> UnicodeDecodeError; duration parse -> ValueError/TypeError/
  KeyError; install check -> OSError/SubprocessError.
- executor: pre_/post_function and progress-polling hooks now use hasattr
  checks, and a real exception inside a defined hook is logged instead of
  swallowed; decode fallbacks and numeric parses narrowed.
- best-effort filesystem ops (makedirs/unlink) -> OSError.
- all backend check_is_installed probes -> OSError/SubprocessError.
- shutdown: also fixes a latent NameError ('failure' was only set in the
  except branch, so a successful logind PowerOff would crash the handler).

No bare 'except:' remain. Full suite (51 tests) green; GUI starts clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/devedeng/shutdown.py

@@ -22,11 +22,12 @@ class shutdown:
     def __init__(self):
 
         # First, try with logind
+        failure = False
         try:
             bus = Gio.bus_get_sync(Gio.BusType.SYSTEM, None)
             bus.call_sync("org.freedesktop.login1", "/org/freedesktop/login1", "org.freedesktop.login1.Manager",
                           "PowerOff", GLib.Variant_boolean('(bb)', (False, False)), None, Gio.DBusCallFlags.NONE, -1, None)
-        except:
+        except Exception:
             failure = True
 
         if (failure):
@@ -37,7 +38,7 @@ class shutdown:
                 bus = Gio.bus_get_sync(Gio.BusType.SYSTEM, None)
                 bus.call_sync("org.freedesktop.ConsoleKit", "/org/freedesktop/ConsoleKit/Manager", "org.freedesktop.ConsoleKit.Manager",
                               "Stop", None, None, Gio.DBusCallFlags.NONE, -1, None)
-            except:
+            except Exception:
                 failure = True
 
         if (failure):
@@ -48,5 +49,5 @@ class shutdown:
                 bus = Gio.bus_get_sync(Gio.BusType.SYSTEM, None)
                 bus.call_sync("org.freedesktop.Hal", "/org/freedesktop/Hal/devices/computer", "org.freedesktop.Hal.Device.SystemPowerManagement",
                               "Shutdown", None, None, Gio.DBusCallFlags.NONE, -1, None)
-            except:
+            except Exception:
                 failure = True