Clean up error handling

Make several changes to error handling:

• Check for negative values from sprintf, and treat them as errors.

• Check for errors from fscanf both by comparing its return value against
  EOF and by checking ferror.

• Avoiding calling perror with errno set to -1.  Either set a meaningful
  value, such as EINVAL, or use fprintf if errno cannot be expected to
  have a meaningful value.

• Instead of re-using errno for internal error handling, use a new variable
  to indicate errors where it makes sense to do so.

• In read_device, print more specific error messages if we fail to read
  a device's current or max brightness, and print an error message
  immediately if a read fails, instead of only maybe at the end of
  read_device.

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