fix: improves panic messages when there is no reactor/timer

[EverlastingBugstopper]

I want to preface this by saying I'm not entirely sure if these error messages are exactly accurate but I wanted to take a crack at it.

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Motivation

Providing a more actionable error message when tokio expects to be within a runtime. Fixes #1713

Solution

New error messages:

When there is no timer:

The timer is shutdown, you must execute from within the tokio runtime

When there is no reactor:

There is no reactor, you must execute from within the tokio runtime

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I'm not in love with the "you must execute from within the tokio runtime", not sure what the best wording there is, would love some pointers :)

[carllerche]

Thanks for getting this going. Unfortunately, the code has changed significantly since the PR was open.

What about something like:

"called from outside of the Tokio runtime or from a runtime that has I/O disabled."

[LucioFranco]

I think we'd love to have this but the code has changed a lot. I am going to close this but we should open a new one against the current version of the code base :)