gh-127257: Add hex code to ssl "unknown error" message (GH-127360)

* ssl: Add hex error code to "unknown error" messages

To make it easier to vary the individual parts of the message,
replace the if-ladder with constant format strings by building
the string piece-wise with PyUnicodeWriter.

Use "unknown error (0x%x)" rather than just "unknown error" if we
can't get a better error message. (Hex makes sense as the error
includes two packed parts.)