Need better error message when source file does not exist

[erikd]

When ddc is called like:

bin/ddc -c whatever/does_not_exist.ds

for a file that doesn't exist, we get an ugly error message like:

ddc: user error (Pattern match failure in do expression at src/Main.hs:140:1-4)

We need something better.

[erikd]

Fixed in this commit:

Wed Nov 4 20:05:47 EST 2009 Erik de Castro Lopo <erikd@…>

• Fix #144 : Give better error message when source file does not exist.