Slightly friendlier output when looking for Dune module dependencies

• Do not print warnings for optional dependencies which are simply not found (this is the common case)
• Add some spaces to emphasize the other warnings and errors
• Fix some trailing whitespace

Note that this keeps the WARNING when there is a version mismatch with an optional dependency because then probably something is wrong in the setup.

Additionally, add the parameter -v|--verbose to the dunecontrol script to re-enable verbose output of warnings.