Media Player error handling

*Inform the user of an unknown filename extension.
*Don't show error messages in OpenMciDevice. Instead show them in the calling function.
*Remove the last hardcoded MCI-specific error message.
*Let OpenMciDevice and CloseMciDevice return 0 instead of TRUE if all goes well.

Also in a previous patch I have set the buffer length needed by DragQueryFile to the size in bytes
instead the size in characters, so fix this also.