Protect against NPE and improve error message

Update ConfigurationMetadataAnnotationProcessor so that `prefix` is
only obtained when the annotation is not null. Also improve exception
message by including the element.

spring-boot-tools/spring-boot-configuration-processor/src/main/java/org/springframework/boot/configurationprocessor/ConfigurationMetadataAnnotationProcessor.java

@@ -101,7 +101,8 @@ public class ConfigurationMetadataAnnotationProcessor extends AbstractProcessor
 		super.init(env);
 		this.typeUtils = new TypeUtils(env);
 		this.metadataStore = new MetadataStore(env);
-		this.metadataCollector = new MetadataCollector(env, this.metadataStore.readMetadata());
+		this.metadataCollector = new MetadataCollector(env,
+				this.metadataStore.readMetadata());
 		try {
 			this.fieldValuesParser = new JavaCompilerFieldValuesParser(env);
 		}
@@ -128,10 +129,11 @@ public class ConfigurationMetadataAnnotationProcessor extends AbstractProcessor
 	}
 
 	private void processElement(Element element) {
+		try {
 			AnnotationMirror annotation = getAnnotation(element,
 					configurationPropertiesAnnotation());
-		String prefix = getPrefix(annotation);
 			if (annotation != null) {
+				String prefix = getPrefix(annotation);
 				if (element instanceof TypeElement) {
 					processAnnotatedTypeElement(prefix, (TypeElement) element);
 				}
@@ -140,6 +142,11 @@ public class ConfigurationMetadataAnnotationProcessor extends AbstractProcessor
 				}
 			}
 		}
+		catch (Exception ex) {
+			throw new IllegalStateException(
+					"Error processing configuration meta-data on " + element, ex);
+		}
+	}
 
 	private void processAnnotatedTypeElement(String prefix, TypeElement element) {
 		String type = this.typeUtils.getType(element);