add log message MOAI_FileOpenError_S

While this may seem redundant with MOAI_FileNotFound_S, there
may be cases where the file exists but can't be opened, e.g.,
due to permission errors. This also supports a safer code structure:
actually try to open the file and log if it fails, instead of checking
for file existence and then just assuming it will open.
use MOAILogMessages::CheckFileExists instead of USFileSys::CheckFileExists so we get an error message
Log if file didn't open successfully
check and log if sound file doesn't exist

I would have preferred to check whether the actual file-open
operation succeeded, but this would require adding a check inside
untz itself, which doesn't have a clear logging mechanism at
the moment.
- Camera support for iOS on moai
- Fixed not dimissing picker
added priority to USTask, USTaskThread; renamed USTask publication callback to clarify intent
decided to go with 'CompletionDelegate' instead of 'PublicationDelegate' in USTask
broke USTaskThread into USTaskThread and USTaskSubscriber; updated MOAISim to use new model
added missing USTaskSubscriber.cpp and .h
moved all thread classes to moaicore, renamed from US* to MOAI*
merged MOAIData into MOAIDataBuffer
factored MOAITasksQueue out of MOAITaskThread; merged MOAIDataIOAction with MOAIDataIOTask; updated MOAIDataIOTask to use new model
added support for default texture; finished async load; samples for async texture load and default texture
added support in MOAIDataBuffer to detect zip files, inflate on load; added inflate on load support to MOAIDataIOTask
exposed inflate on load flags from MOAIDataBuffer
fixed inflate on load behavior in MOAIDataIOTask
Adding file not found log if a texture file fails to load
Fixing build errors for DataTaskIO
fix: MOAIImageTexture not override OnInvalidate