[BrowserKit] Throw exception on invalid cookie expiration timestamp

Currently, if an invalid timestamp is provided, a fatal error with no stack trace will occur, making it difficult to trace the problem.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        |