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
IPath path= getOutputLocation();
		
		IResource res= fWorkspaceRoot.findMember(path);
		if (res != null) {
			
			if (res.getType() == IResource.FILE) {
				fBuildPathStatus.setError(NewWizardMessages.getString("BuildPathsBlock.error.InvalidBuildPath")); 
				return;
			}
		}
		fOutputLocationPath= path;
		
		List elements= fClassPathList.getElements();
		IClasspathEntry[] entries= new IClasspathEntry[elements.size()];
		
		for (int i= elements.size()-1 ; i >= 0 ; i--) {
			CPListElement currElement= (CPListElement)elements.get(i);
			entries[i]= currElement.getClasspathEntry();
		}
		
		IStatus status= JavaConventions.validateClasspath(fCurrJProject, entries, path);
		if (!status.isOK()) {
			fBuildPathStatus.setError(status.getMessage());
			return;
		}
		if (res != null && res.exists() && fCurrJProject.exists()) {
			try {
				IPath oldOutputLocation= fCurrJProject.getOutputLocation();
				if (!oldOutputLocation.equals(fOutputLocationPath)) {
					if (((IContainer)res).members().length > 0) {