Correctly handle precedence of ToolsVersion value coming

Correctly handle precedence of ToolsVersion value coming
from various points.
In class/Microsoft.Build.Engine:

	* Microsoft.Build.Engine.dll.sources: Add
	UnknownToolsVersionException.cs

In class/Microsoft.Build.Engine/Microsoft.Build.BuildEngine:

	Correctly handle precedence of ToolsVersion value coming
	from various points.
	* BuildEngine.cs (BuildProjectFile): Restore project.ToolsVersion .
	* Engine.cs: Likewise. Also validate DefaultToolsVersion .
	* Project.cs: Handle invalid tools version specified in the project
	file, and fall back to engine's version.
	* MSBuild.cs: Fix the precedence order, and validate.
	* UnknownToolsVersionException.cs: New.

In class/Microsoft.Build.Tasks:

	* Microsoft.Build.Tasks.dll.sources: Use
	UnknownToolsVersionException.cs from Engine.

In class/Microsoft.Build.Tasks/Test/Microsoft.Build.Tasks:

	* MessageTest (TestExecution): Additional messages with 'low'
	importance get emitted, so don't expect sequential messages.

In tools/xbuild:

	* Main.cs: Use tools version specified on the command line, if
	any.
	* xbuild.exe.sources: Include UnknownToolsVersionException.cs from
	MS.B.Engine

svn path=/trunk/mcs/; revision=160133