Lower severity of m2e's Project configuration is not up-to-date... diagnostics

[fbricon]

Currently, the build command fails whenever the Project configuration is not up-to-date with pom.xml. Select: Maven->Update Project... from the project context menu or use Quick Fix. problem is reported on a Maven project.
We have 2 issues here:

• First of all, the severity should be downgraded to warning in the server preferences, as it's rarely an actual error
• The message itself is completely Eclipse-UI bound, thus irrelevant to other clients. It should be overridden and reported to clients as Project configuration is not up-to-date with pom.xml, requires an update. (a better message will be accepted).

See microsoft/vscode-java-debug#358 and redhat-developer/vscode-java#608