Only consider existing artifacts for the classpath

Currently only if the artifact file is null it is not considered but if
the file does not exits JDT complains. It even seems there was once in a
time such a check but it is disabled, this is an attempt to check if we
can reenable it again.

org.eclipse.m2e.jdt/src/org/eclipse/m2e/jdt/internal/DefaultClasspathManagerDelegate.java

@@ -137,7 +137,7 @@ void addClasspathEntries(IClasspathDescriptor classpath, IMavenProjectFacade fac
 
       } else {
         File artifactFile = a.getFile();
-        if(artifactFile != null /*&& artifactFile.canRead()*/) {
+        if(artifactFile != null && artifactFile.exists()) {
           entry = classpath.addLibraryEntry(IPath.fromOSString(artifactFile.getAbsolutePath()));
           entry.setClasspathAttribute(IClasspathManager.TEST_ATTRIBUTE, addTestFlag ? "true" : null);
         }