Add precondition check when scanning

[fabapp2]

What needs to be done

A set of preconditions should be run at the beginning of a scan to warn the user about
potential problems when scanning an application that does not fulfill requirements.

Why it needs to be done

For a better UX a user should be warned about expected problems when scanning (but before applying recipes) an application that does not fulfill given preconditions.

Acceptance Criteria

Given A user scans an application that does not fulfill one or more preconditions +
When SBM performs a precondition check that fails +
Then A warning is shown to the user and the scan is cancelled.

Precondition	Warning
No pom.xml found	SBM requires a Maven build file. Please provide a minimal pom.xml.
no dir src/main/java exists	Precondition check could not find a src/main/java dir. This dir is required.
Git-Support is enabled but no .git exists	Git-Support is enabled but the scanned project is not under git.
Multiple modules found	Maven reactor support is experimental and not fully supported.

Additional Information