Signal an error if the root directory does not exist

* indium-workspace.el (indium-workspace-root): Signal an error is the root
  directory is invalid (non-existent or not a directory).
* test/unit/indium-workspace-test.el: Add a regression test.

indium-workspace.el

@@ -112,10 +112,15 @@ If no connection is open yet, lookup the root directory as follows:
     \"root\") configuration option.
 
   - If no \"root\" option is set, it defaults to the directory
-    containing the \".indium.json\" project file."
-  (or (indium-current-connection-project-root)
-      (indium-workspace--root-from-configuration)
-      (indium-workspace--project-directory)))
+    containing the \".indium.json\" project file.
+
+If the root directory does not exist, signal an error."
+  (let ((root (or (indium-current-connection-project-root)
+		  (indium-workspace--root-from-configuration)
+		  (indium-workspace--project-directory))))
+    (unless (file-directory-p root)
+      (user-error "Project root directory does not exist"))
+    root))
 
 (defun indium-workspace--root-from-configuration ()
   "Return the root directory read from the project configuration.

test/unit/indium-workspace-test.el

@@ -34,26 +34,36 @@
 
 (describe "Workspace root"
   (it "Returns the current connection's project root when there is a connection"
-    (let ((indium-current-connection (make-indium-connection :project-root 'foo)))
-      (expect (indium-workspace-root) :to-be 'foo)))
+    (assess-with-filesystem indium-workspace--test-fs
+      (let* ((root (expand-file-name "js"))
+	     (indium-current-connection (make-indium-connection :project-root root)))
+	(expect (indium-workspace-root) :to-be root))))
 
   (it "should default to the project directory when no \"root\" is defined"
     (assess-with-filesystem indium-workspace--test-fs
       (expect (directory-file-name (indium-workspace-root)) :to-equal
 	      (directory-file-name default-directory))))
 
   (it "should take the directory set in the \"root\" option"
-    (assess-with-filesystem '((".indium.json" "{\"configurations\": [{\"root\": \"foo\"}]}"))
+    (assess-with-filesystem '((".indium.json" "{\"configurations\": [{\"root\": \"foo\"}]}")
+			      ("foo" ("index.js")))
       (with-indium-workspace-configuration
 	(expect (directory-file-name (indium-workspace-root)) :to-equal
 		(directory-file-name (expand-file-name "foo" default-directory))))))
 
   (it "webRoot should be an alias for root"
-    (assess-with-filesystem '((".indium.json" "{\"configurations\": [{\"webRoot\": \"foo\"}]}"))
+    (assess-with-filesystem '((".indium.json" "{\"configurations\": [{\"webRoot\": \"foo\"}]}")
+			      ("foo" ("index.js")))
       (with-indium-workspace-configuration
 	(expect (directory-file-name (indium-workspace-root)) :to-equal
 		(directory-file-name (expand-file-name "foo" default-directory)))))))
 
+(describe "Invalid root directory"
+    (it "should signal an error when the root directory does not exist"
+    (assess-with-filesystem '((".indium.json" "{\"configurations\": [{\"webRoot\": \"foo\"}]}"))
+      (with-indium-workspace-configuration
+	(expect (indium-workspace-root) :to-throw)))))
+
 (describe "Choosing a configuration"
     (it "should not prompt for a configuration when there is only one"
       (assess-with-filesystem '((".indium.json" "{\"configurations\": [{}]}"))