Improve exception messages on nested jar failure

Updates gh-284

spring-boot-tools/spring-boot-loader/src/main/java/org/springframework/boot/loader/jar/JarFile.java

@@ -305,11 +305,17 @@ public class JarFile extends java.util.jar.JarFile implements Iterable<JarEntryD
 	 */
 	public synchronized JarFile getNestedJarFile(final JarEntryData sourceEntry,
 			JarEntryFilter... filters) throws IOException {
+		try {
 			if (sourceEntry.isDirectory()) {
 				return getNestedJarFileFromDirectoryEntry(sourceEntry, filters);
 			}
 			return getNestedJarFileFromFileEntry(sourceEntry, filters);
 		}
+		catch (IOException ex) {
+			throw new IOException("Unable to open nested jar file '"
+					+ sourceEntry.getName() + "'", ex);
+		}
+	}
 
 	private JarFile getNestedJarFileFromDirectoryEntry(JarEntryData sourceEntry,
 			JarEntryFilter... filters) throws IOException {