Adapt to current error format (#1397)

As CSharpier currently does not output error messages in MSBuild
compatible format, adapt Exec options to current error format. Should
fix #1357.

Author: PetSerAl

Src/CSharpier.MsBuild/build/CSharpier.MsBuild.targets

@@ -25,6 +25,8 @@
       StdOutEncoding="utf-8"
       StdErrEncoding="utf-8"
       IgnoreExitCode="true"
+      IgnoreStandardErrorWarningFormat="true"
+      CustomErrorRegularExpression="^Error "
       Command=""$(CSharpier_dotnet_Path)" exec "$(CSharpierDllPath)" $(CSharpierArgs) --no-msbuild-check --compilation-errors-as-warnings "$(MSBuildProjectDirectory)" > $(NullOutput) " />
   </Target>