How do I cause an error in MSBuild if a file exists

<CreateProperty Condition="Exists('C:ProcessFail.txt')"
      Value="false">
  <Output TaskParameter="Value" PropertyName="ProcessTestPassed"/>
</CreateProperty>
<Message Text="Process did not pass" Condition="Exists('C:ProcessFail.txt')" ContinueOnError="false" />

<ReadLinesFromFile File="C:ProcessFail.txt"                  Condition="'$(ProcessTestPassed)'=='false'" ContinueOnError="false" >
  <Output TaskParameter="Lines" ItemName="FileContents" />
</ReadLinesFromFile>
<Message Text="FileContents: $(FileContents)"  Condition="'$(ProcessTestPassed)'=='false'" ContinueOnError="false" />

Task "CreateProperty"
Done executing task "CreateProperty".
Task "Message"
  QAWizardProTestPassed did not pass
Done executing task "Message".
Task "ReadLinesFromFile"
Done executing task "ReadLinesFromFile".
Task "Message"
  FileContents:
Done executing task "Message".

<Error Condition="Exists('C:ProcessFail.txt')" Text="Process did not pass!" />