Make errno/strerror/filename available on IOError

An error reporting flaw was that most raised IOErrors got their
errno/strerror/filename fields stripped, due to wanting to show a custom
messages. The problem was that adding back 'errno' and 'strerror' made
IOError.__str__() always return a fixed string
("[Errno <errno>] <strerror>"), ignoring any custom message.

This is friendly to users, but unfriendly to scripts (the menuconfig had
a workaround). Make things friendly to both by raising an internal
subclass of IOError instead, that preserves errno/strerror/filename but
prints a custom message. The exception can then still be caught as
IOError/OSError by scripts.

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