Firefox build needs Dryice 0.4.11

It has needed the filenameFilter feature for quite a while, but since
dryice 0.4.11 has been published to npm we can now depend on it from
package.json.

Signed-off-by: Joe Walker <jwalker@mozilla.com>
1007006-spawn: Allow wider set of messages in exception tests

If Task.spawn catches an exception message then it adds "Error: " onto
the front, so allow a slightly wider set of error messages when testing.

Signed-off-by: Joe Walker <jwalker@mozilla.com>