No bare except clauses

### Motivation
There are numerous bare except clauses (`except:`) throughout the codebase. These clauses catch *all* kinds of exceptions, even exceptions which *aren't* meant to be caught, such as [KeyboardInterrupt](https://docs.python.org/2.7/library/exceptions.html#exceptions.KeyboardInterrupt) and [SystemExit](https://docs.python.org/2.7/library/exceptions.html#exceptions.SystemExit). This is bad style and a possible source of errors.

### Changelog
* Activate `pylint` flag `W0702` flagging bare except clauses.
* Rewrite all bare `except:` clauses as `except Exception:`.

### Roadmap
Since `Exception` is the ancestor of all "catchable" exceptions, its type doesn't document what kind of error has happened. We should try to further restrict ourselves by catching only the actual exception raised wherever possible. See `pylint` flag `W0703`. For more information about the Python exception hierarchy, see the [docs](https://docs.python.org/2.7/library/exceptions.html).