Bug 9159 Classpath wizard should not complain when adding missing source folder
Build 20020125 - Define a project A with source folder 'src1'. - Go to the .classpath file and edit it to insert one extra line: <classpathentry kind="src" path="src2"/> below the line referring to first source folder: <classpathentry kind="src" path="src1"/>. - Now, try to add this missing source folder 'src2', it isn't allowed by the wizard, one has to add a basic folder instead. Note: It is not such an uncommon scenario, the classpath modification could simply occur by catching up with a shared classpath having one more source folder on it, and which is locally missing.
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/wizards/buildpaths/BuildPathsBlock.java
} catch (CoreException e) {
			JavaPlugin.log(e.getStatus());
		}
		if (outputLocation == null) {
			outputLocation= getDefaultBuildPath(jproject);
		}
		
		List newClassPath;
		if (classpathEntries == null) {
			newClassPath= getDefaultClassPath(jproject);
		} else {
			newClassPath= new ArrayList();
			for (int i= 0; i < classpathEntries.length; i++) {
				IClasspathEntry curr= classpathEntries[i];
				int entryKind= curr.getEntryKind();
				IPath path= curr.getPath();
				boolean isExported= curr.isExported();
				
				IResource res= null;
				boolean isMissing= false;
				
				if (entryKind != IClasspathEntry.CPE_VARIABLE) {
					res= fWorkspaceRoot.findMember(path);
					if (res == null) {
						isMissing= (entryKind != IClasspathEntry.CPE_LIBRARY || !path.toFile().isFile());
					}
				} else {
					IPath resolvedPath= JavaCore.getResolvedVariablePath(path);
					isMissing= (resolvedPath == null) || !resolvedPath.toFile().isFile();
				}