STY/TST: minor style fix and new tests in `timeseries`

[fkuehlein]

• correct bad linebreaks in exception messages, blame e1fb60b
• test for correct raising of modified exceptions in JointRecurrencePlotto make CodeCov happy
• also add basic test for JointRecurrenceNetwork for improved coverage, inspired by #72

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 68.71%. Comparing base (d63cbb7) to head (e469586).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #231 +/- ## ========================================== + Coverage 68.63% 68.71% +0.08% ========================================== Files 44 44 Lines 6138 6138 ========================================== + Hits 4213 4218 +5 + Misses 1925 1920 -5 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[fkuehlein]

Just realized my test for JointRecurrenceNetwork covers no more than tests/test_generic.py already does, so overall coverage is hardly improved. Still adding some basic assertions though, and a test that might be extended some time.