export: Reject custom target exports earlier (#15657)

Diagnose and reject custom targets given to the export() command
immediately.  Previously we would generate an internal error later.

Source/cmExportCommand.cxx

@@ -177,6 +177,12 @@ bool cmExportCommand
           this->SetError(e.str());
           return false;
           }
+        if (target->GetType() == cmTarget::UTILITY)
+          {
+          this->SetError("given custom target \"" + *currentTarget
+                         + "\" which may not be exported.");
+          return false;
+          }
         }
       else
         {

Tests/RunCMake/export/CustomTarget-result.txt

+1

Tests/RunCMake/export/CustomTarget-stderr.txt

+^CMake Error at CustomTarget.cmake:[0-9]+ \(export\):
+  export given custom target "CustomTarget" which may not be exported.
+Call Stack \(most recent call first\):
+  CMakeLists.txt:3 \(include\)$

Tests/RunCMake/export/CustomTarget.cmake

+add_custom_target(CustomTarget)
+export(TARGETS CustomTarget FILE somefile.cmake)

Tests/RunCMake/export/RunCMakeTest.cmake

 include(RunCMake)
 
+run_cmake(CustomTarget)
 run_cmake(TargetNotFound)
 run_cmake(AppendExport)
 run_cmake(OldIface)