Check dependencies for a better error

[panglesd]

When a dependency is missing, the error message is never informative.

We should check existence of dependencies at startup to directly warn the user about missing dependencies.

[Julow]

For the external dependencies of the tools, opam depext should have nice messages and can also install them on distros where it knows about the package manager. It's also future proof, we don't need to hardcode a list of packages and we don't have to deal with packages that have different names on different distros.
For the dependencies of the installer script, we need a clear error message.