prevent parallel plot requests

Plotting can take a long time and use a lot of resources. 
Multiple plot requests can cause the machine to run OOM.

We are now allowing plots for 500k files again. This is mainly to
prevent unwanted plots of everything.

pdb-ui/src/main/java/org/lucares/pdbui/ServiceUnavailableException.java

@@ -0,0 +1,14 @@
+package org.lucares.pdbui;
+
+import org.springframework.http.HttpStatus;
+import org.springframework.web.bind.annotation.ResponseStatus;
+
+@ResponseStatus(value = HttpStatus.SERVICE_UNAVAILABLE, reason = "Service Unavailable")
+public class ServiceUnavailableException extends RuntimeException {
+
+	private static final long serialVersionUID = -4512668277873760500L;
+
+	public ServiceUnavailableException(String message) {
+		super(message);
+	}
+}