MOBILE-3062 - Basic validation and error handling for folder creation

At Alfresco - and mostly for historic reasons - we use the [Allman style](http://en.wikipedia.org/wiki/Indent_style#Allman_style) for code braces (and parentheses). In other words, all open braces are given a new line:
```objectivec
(BOOL)isValidFolderName:(NSString *)folderName
{
   ...
}

There are some exceptions to this rule when using completion blocks for readability, but most code needs to follow this style.