Better error when phase contains non-plugin declarations

[scorfield]

A user did this by accident:

<phase name="postProcess">
<do action="layouts.buildPage"/>
</phase>

The error was:

Required attribute is missing
The attribute 'name' is required, for a 'postProcess' plugin declaration in fusebox.xml.

It would be more helpful to complain that 'do' is not valid inside a 'phase' declaration.

[scorfield]

This is a no brainer. 5.1 should absolutely do this.

[scorfield]

(In [262]) Fixes #158 by moving the attribute check for plugins inside the (existing) conditional that checked the declaration's xmlName.