raise text value instead of exception

[Robert Gildein]

I found that there are two exceptions (on these lines [1] and [2]) that raise the text value instead of some Exception. This will raise an error (`TypeError: exceptions must derive from BaseException`), but not the one expected.

---
[1]: https://github.com/openstack/charm-ceph-osd/blob/785ee7c455a58b9ffea1c45b59750a85180ea59d/lib/charms_ceph/crush_utils.py#L84
[2]: https://github.com/openstack/charm-ceph-osd/blob/785ee7c455a58b9ffea1c45b59750a85180ea59d/lib/charms_ceph/crush_utils.py#L114

[Robert Gildein (rgildein)]

review: https://review.opendev.org/c/openstack/charms.ceph/+/780226

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/c/openstack/charms.ceph/+/780226
Committed: https://opendev.org/openstack/charms.ceph/commit/1a7e8df4efc3a0c27b5a67a4af49dc107087da4e
Submitter: "Zuul (22348)"
Branch: master

commit 1a7e8df4efc3a0c27b5a67a4af49dc107087da4e
Author: Robert Gildein <email address hidden>
Date: Fri Mar 12 12:19:34 2021 +0100

    Fixed raise text value instead of Exception

    I used ValueError for both exception.
    Also removed unnecessary flake8 ignore option.

    Closes-Bug: #1917351
    Change-Id: I5ee58aa0a34d7c266b3811f9c6b9b3076bfdc048