TransferElement::isReadable()/isWriteable() and exceptions

[mhier]

The spec is a bit vague about TransferElement::isReadable()/isWriteable() throwing exceptions. C.5.3.4 says merely "If the test function needs to obtain the information from the device, a runtime_error might occur.". This needs to be extended so calling isReadable()/isWriteable() throws both logic_error and runtime_error under similar conditions as read or write operations:

• If the device is closed, a logic_error must be thrown
• If setException() has been called before, a runtime_error must be thrown
• In addition, what C.5.3.4 currently says, if isReadable()/isWriteable() need to communicate because no cached information or information from a map file is available, a runtime_error might occur if the device is broken.

This requires changes in the spec, in the implementations and in the UnifiedBackendTest.