snapuserd: Return void from RespondIOError.

RespondIOError could return "true" which is not the correct value for
its callers, usually. However since RespondIOError is not specifically
needed anymore, we can also avoid calling it in most places.

This also fixes a bug where ReadUnalignedSector's return value was
implicitly converted to boolean.

Bug: 288273605
Test: snapuserd_test
Change-Id: I62140b2b05d0f9f53cb669c5c0d7e0ffc7f4c9a1