Better arrival-rate message when it can't hit rate

[mstoykov]

Feature Description

Currently if an arrival-rate scenario can't hit the provided rate there is a single warning message. https://github.com/grafana/k6/blob/f3fb49300cb8dba7558b8e841a80f7212a549cba/lib/executor/ramping_arrival_rate.go#L483

While this message is okay in general there are a lot of users who do not pay it much attention or do not understand that this means the SUT was not fast enough or preallocatedVUs/maxVUs was set too low.

Suggested Solution (optional)

For starters maybe rewrite the message to make it more clear that this very likely is a problem they should look into.

Maybe move it to an error instead of a warning - like this literally means the configured scenario could not do what it was supposed to do

Already existing or connected issues / PRs (optional)

No response