Validate task messages

[MetRonnie]

Update: see #3820 for follow on.

These changes close #3428

Task messages, e.g. in

[runtime]
    [[my_task]]
        [[[outputs]]]
            foo = I am an invalid task message: the colon is to blame

can only have colons in them if the first part of the message is a logging severity level plus colon, e.g. INFO:, because everything before the first colon is treated as a severity level.

This PR causes validation to fail if colons are misused.

Requirements check-list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).

• Appropriate tests are included (unit).

• Appropriate change log entry included.

• No documentation update needed (superseded by Validate task messages & task outputs separately #3820)

• No dependency changes.

[oliver-sanders]

Looks good, tested as working.