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/avconv.py

@@ -103,7 +103,7 @@ class avconv(devedeng.avbase.avbase):
                 return True
             else:
                 return False
-        except:
+        except (OSError, subprocess.SubprocessError):
             return False
 
     def __init__(self):
@@ -648,7 +648,7 @@ class avconv(devedeng.avbase.avbase):
                 for e in parts:
                     try:
                         v = float(e)
-                    except:
+                    except ValueError:
                         continue
                     t = t * 60.0 + v
                 t /= self.final_length