Error out if dlls can't be found, and reorder locations where we go look for them

Author: shana

common/unitylocation.props

@@ -1,9 +1,15 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <PropertyGroup>
-    <UnityDir Condition="$(UnityDir) == '' and Exists('C:\Program Files (x86)\Unity\Editor\Data\Managed\UnityEditor.dll')">C:\Program Files (x86)\Unity\Editor\Data\Managed\</UnityDir>
-    <UnityDir Condition="$(UnityDir) == '' and Exists('\Applications\Unity\Unity.app\Contents\Managed\UnityEditor.dll')">\Applications\Unity\Unity.app\Contents\Managed\</UnityDir>
     <UnityDir Condition="$(UnityDir) == '' and Exists('$(SolutionDir)\script\lib\UnityEditor.dll')">$(SolutionDir)\script\lib\</UnityDir>
     <UnityDir Condition="$(UnityDir) == '' and Exists('$(SolutionDir)\lib\UnityEditor.dll')">$(SolutionDir)\lib\</UnityDir>
+    <UnityDir Condition="$(UnityDir) == '' and Exists('C:\Program Files (x86)\Unity\Editor\Data\Managed\UnityEditor.dll')">C:\Program Files (x86)\Unity\Editor\Data\Managed\</UnityDir>
+    <UnityDir Condition="$(UnityDir) == '' and Exists('\Applications\Unity\Unity.app\Contents\Managed\UnityEditor.dll')">\Applications\Unity\Unity.app\Contents\Managed\</UnityDir>
   </PropertyGroup>
+  <Target Name="BeforeBuild">
+    <PropertyGroup>
+      <ErrorText>Location of Unity dlls is not set. You'll need to install Unity in a known location (the default installation directory for your system), or copy UnityEngine.dll and UnityEditor.dll to the {0}lib folder</ErrorText>
+    </PropertyGroup>
+    <Error Condition="'$(UnityDir)' == ''" Text="$([System.String]::Format('$(ErrorText)', '$(SolutionDir)'))" />
+  </Target>
 </Project>