[MNG-8349] Avoid exceptions with invalid modelVersion

[gnodet]

JIRA issue: MNG-8349

The output is now the expected one (albeit duplicated):

[INFO] Scanning for projects...
[ERROR] Some problems were encountered while processing the POMs
[ERROR] The build could not read 1 project -> [Help 1]
[ERROR]   
[ERROR]   The project org.test:foo:jar:1.0-SNAPSHOT (/Users/gnodet/tmp/MNG-8189/pom.xml) has 2 errors
[ERROR]     'modelVersion' must be one of [4.0.0, 4.1.0] but is '4..0.0'. @ org.test:foo:1.0-SNAPSHOT, file:///Users/gnodet/tmp/MNG-8189/pom.xml, line 2, column 3
[ERROR]     'modelVersion' must be one of [4.0.0, 4.1.0] but is '4..0.0'. @ org.test:foo:1.0-SNAPSHOT, file:///Users/gnodet/tmp/MNG-8189/pom.xml, line 2, column 3
[ERROR] 
[ERROR] To see the full stack trace of the errors, re-run Maven with the '-e' switch
[ERROR] Re-run Maven using the '-X' switch to enable verbose output
[ERROR] 
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/ProjectBuildingException