Replace AssertionErrors with RuntimeExceptions

[acogoluegnes]

Use e.g. IllegalStateException instead of AssertionError in BlockingCell and other part of the code (when appropriate). The Error family is too dramatic for this class and client code that tries to handle exceptions from BlockingCell could end up dealing with cases it cannot recover from anyway.

Related to #237.

Reverse also the changes from #237, so the NIO loop doesn't actually handle AssertionError coming from elsewhere than BlockingCell.

[acogoluegnes]

See suggestions from @dimas:
#237 (comment)
#237 (comment)

[vikinghawk]

@acogoluegnes

Looks like there are still some instances of Error's in the code base such as UnexpectedFrameError, UnexpectedMethodError, and some places that throw a generic Error. Any chance those places could be switched to RuntimeExceptions too? For example there are a couple places that catch UnsupportedEncodingExceptions and wrap them in an Error. The code could be updated to use Charset.forName() which throws a runtime UnsupportedCharsetException.

[michaelklishin]

@vikinghawk we would definitely consider that for 5.0. Feel free to submit a PR against master.

[acogoluegnes]

Fixed by 3c010f7. Thanks @vikinghawk !