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
Fixing build errors for DataTaskIO
fix: MOAIImageTexture not override OnInvalidate
Merge pull request #619 from antback/master

Initial Camera Support for iOS
Merge pull request #618 from tommo/bug-fix

fix: MOAIImageTexture not override OnInvalidate
Merge pull request #592 from sporksmith/log-file-open-fails-pull-req

Log file-open failures
Merge remote-tracking branch 'upstream/master'