plugin/java: handle missing argument

Provides an error message if no file is specified.

plugins/available/java.plugin.bash

@@ -2,10 +2,10 @@ cite about-plugin
 about-plugin 'Java and JAR helper functions'
 
 function jar_manifest {
-  about "extracts the specified JAR file's MANIFEST file and prints it to stdout"
-  group 'java'
-  param '1: JAR file to extract the MANIFEST from'
-  example 'jar_manifest lib/foo.jar'
+	about "extracts the specified JAR file's MANIFEST file and prints it to stdout"
+	group 'java'
+	param '1: JAR file to extract the MANIFEST from'
+	example 'jar_manifest lib/foo.jar'
 
-  unzip -c "$1" META-INF/MANIFEST.MF
+	unzip -c "${1?${FUNCNAME[0]}: manifest must be specified}" META-INF/MANIFEST.MF
 }