Add nicer error message when SDCard "Lock" switch is engaged

[ghost]

When the "lock" switch is turned on, the error message that gets reported is accurate, but misleading. Add text to ask user to check the switch. Here's an example of the current output:

$ sudo mmccopy image.iso 
Use 3.69 GiB memory card found at /dev/sdc? [y/N] y
mmccopy: /dev/sdc: Read-only file system

[fhunleth]

Fixed.