Null out mainPath on resolve errors

The scope of this variable is outside the loop so this prevent
cache corruption based on using the mainPath value from previous
module.

Refs atom/atom-space-pen-views#5

src/module-cache.coffee

@@ -59,6 +59,8 @@ loadDependencies = (modulePath, rootPath, rootMetadata, moduleCache) ->
     if childMetadata?.version
       try
         mainPath = require.resolve(childPath)
+      catch error
+        mainPath = null
 
       if mainPath
         moduleCache.dependencies.push