Ease transition into using workspace mode by default

[Xanewok]

To make the transition easier, it'd be good to inform user that they opened a workspace, yet still use "workspace_mode": false. Previously it just crashed, corrupting RLS (#598).

I believe a good solution would be to allow builds to return messages that will be displayed for the user. One example might be an information: "Attempted to perform a single-package build inside a Cargo workspace. Consider turning "workspace_mode" option on." to explain users why the code doesn't work and what to do to fix this.
Another use would be to inform user that rustc has encountered an ICE and thus won't give any meaningful diagnostics. Currently this is only shown in RLS' stderr and rls-vscode happily displays RLS Analysis: Done misleading users.

One small step to simplify the BuildResult variant to further contains also notifications is #601.

cc #132

[Xanewok]

In general returning notifications from build might be something we can consider, but currently workspace mode is on by default, so it's not needed now.