Improve cmd error classification; add Git repository support

app/classes/logical/UniversalPath.js

@@ -65,10 +65,16 @@ class UniversalPath extends Injectable {
 
     async parent() {
         await this.classify()
-        if ( this.is_directory() ) return path.resolve(this.path, '..')
+        if ( this.is_directory() ) return path.join(this.path, '..')
         return this.directory()
     }
 
+    async child(subpath) {
+        await this.classify()
+        if ( !this.is_directory() ) throw new Error('Cannot get sub-path: not a directory: '+this.path)
+        return this.constructor.fromString(path.join(String(this), subpath))
+    }
+
     name() {
         return path.basename(this.path)
     }

app/classes/logical/error/FilesystemResourceNotFoundError.js

@@ -0,0 +1,5 @@
+class FilesystemResourceNotFoundError extends Error {
+
+}
+
+module.exports = exports = FilesystemResourceNotFoundError