clarified "isabelle phabricator -l": avoid surprise with non-existent default installation;

--- a/src/Doc/System/Phabricator.thy	Wed Dec 11 23:05:57 2019 +0100
+++ b/src/Doc/System/Phabricator.thy	Thu Dec 12 11:49:00 2019 +0100
@@ -254,7 +254,7 @@
   is the subdirectory \<^verbatim>\<open>phabricator\<close> within the root.
 
   \<^medskip> Option \<^verbatim>\<open>-l\<close> lists the available Phabricator installations with name and
-  root directory.
+  root directory --- without invoking a command.
 
   Option \<^verbatim>\<open>-n\<close> selects the explicitly named Phabricator installation.
 \<close>

--- a/src/Pure/Tools/phabricator.scala	Wed Dec 11 23:05:57 2019 +0100
+++ b/src/Pure/Tools/phabricator.scala	Thu Dec 12 11:49:00 2019 +0100
@@ -154,11 +154,11 @@
           progress.echo("phabricator " + quote(config.name) + " root " + config.root)
         }
       }
-
-      val config = get_config(name)
-
-      val result = progress.bash(Bash.strings(more_args), cwd = config.home.file, echo = true)
-      if (!result.ok) error("Return code: " + result.rc.toString)
+      else {
+        val config = get_config(name)
+        val result = progress.bash(Bash.strings(more_args), cwd = config.home.file, echo = true)
+        if (!result.ok) error("Return code: " + result.rc.toString)
+      }
     })