[error_handler] Small PEP8 fixes.

+ 80 chars on the license.
+ Remove import *
[lib] Remove unused `GetDictValueOrBlank` function.

This functions only does a dict.get(key, '').
So better use `.get()` than `GetDictValueOrBlank`.
[lib] Use `set` instead of `GetUnique`.

+ Remove `GetUnique`
+ Replace old references to `GetUnique` by `set`