#17 No tilde expension in path names

[wren ng thornton]

In path names to files in the paperboyd config format there's no tilde
expansion and only somewhat cryptic error messages are given. As an
ad-hoc measure we should introduce a new error message and test for
file names beginning with "~'. And we should look into how to get tilde
expansion to work. (N.B. it's the shell that normally does it between what
we type and what the new process receives)