Add ability to specify level prototype as a name (close #367, BEM-949)

lib/level.js

@@ -4,14 +4,35 @@ var PATH = require('./path'),
     FS = require('fs'),
     INHERIT = require('inherit'),
     createTech = require('./tech').createTech,
+    util = require('util'),
     bemUtil = require('./util'),
     LOGGER = require('./logger'),
     isRequireable = bemUtil.isRequireable,
 
-    getLevelClass = function(path, optional) {
-        var level = optional && !isRequireable(path) ? {} : requireLevel(path);
+    getLevelClass = function(path, optional, stack) {
+        stack = stack || [];
+
+        var level;
+        try {
+            level = optional && !isRequireable(path) ? {} : requireLevel(path);
+        } catch(error) {
+            throw new Error(util.format(
+                'level module %s can not be found but required by the level %s%s\n',
+                path,
+                stack.length > 1? 'inheritance tree:\n\t': '',
+                stack.join('\n\t')));
+        }
+
+        stack.push(path);
+
         if (level.Level) return level.Level;
-        return INHERIT(level.baseLevelPath? getLevelClass(level.baseLevelPath) : Level, level);
+
+        var baseLevelPath = level.baseLevelPath ||
+            (level.baseLevelName?
+            PATH.join(PATH.resolve(__dirname, './levels'), level.baseLevelName):
+            null);
+
+        return INHERIT(baseLevelPath? getLevelClass(baseLevelPath, false, stack) : Level, level);
     },
 
     requireLevel = function(path) {