the errors should have a better spelling

Currently, the error look like this:

NIOHTTP2Errors.NoSuchStream(...)

but I think it should be either

NIOHTTP2Error.noSuchStream(...) // imitating an open enum on top of a struct

or

NIOHTTP2Errors.NoSuchStreamError(...)

IMHO, preferably the former

Start by locating the NIOHTTP2Errors.NoSuchStream declaration and all of its uses. Check how the existing error cases are exposed and tested, then apply the preferred NIOHTTP2Error.noSuchStream spelling consistently; done means the public error API and its references use the agreed naming.

Written by the indexing model from the issue text.