Tell users exactly what went wrong. Be specific—vague error messages
The specified directory exists but is not writable. To add files to this
directory, the directory must be writable. [Explanation of how to make this
Invalid field 'picture'.
The 'picture' field can only appear once on the command line; this command
line contains the 'picture' field <N> times.
Note: Prior to version 2.1, you could specify the 'picture' field more than
once, but more recent versions no longer support this.