Non AsyncContext & map-like contexts should fail fast

Currently, if a context is neither, it will do something undefined, most likely fails, but could just do nothing. If it's neither, we should blow up in a manged way, relevant code here: https://github.com/metosin/sieppari/blob/develop/src/sieppari/core.cljc#L36-L43

This happens for example when someone returns a `core.async` channel, without having it registered as a valid AsyncContext.

Start with src/sieppari/core.cljc lines 36-43, where non-AsyncContext and map-like contexts are handled. Trace the example of an unregistered core.async channel and define the managed failure for values matching neither context type. Done means unsupported contexts fail explicitly instead of being ignored or behaving undefinedly.

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