Attempt to create an existing module should have better error message

[blee]

blee@util8.sjc1 ~/src/conf $ confman create kern1.lab2
confman[53587]: blee: svn: Try 'svn add' or 'svn add --non-recursive' instead?
confman[53587]: blee: svn: Can't create directory '/home/blee/src/conf/kern1.lab2': File exists
confman[53587]: blee: svn: Try 'svn add' or 'svn add --non-recursive' instead? 
confman[53587]: blee: svn: Can't create directory '/home/blee/src/conf/checkpoints/kern1.lab2': File exists

There should be a confman error message, like "module already exists", rather than an svn error message.

[blee]

(In [430]) Improve error handling for conf_create_module.

See #67

[blee]

(In [431]) Better error message for conf_create_module.

See #67

[blee]

Now with an error message and appropriate exit status:

blee@supra.b1c1l1 ~/src/conf $ confman create gentoo
confman[5605]: blee: Module already exists: gentoo
blee@supra.b1c1l1 ~/src/conf $ echo $?
1

[blee]

(In [432]) Merge from trunk improved error handling for conf_create_module.

Fixes #67