install(EXPORT): Improve target import failure message format

Source/cmExportFileGenerator.cxx

@@ -385,19 +385,16 @@ cmExportFileGenerator::GenerateImportedFileCheckLoop(std::ostream& os)
         "FOREACH(target ${_IMPORT_CHECK_TARGETS} )\n"
         "  FOREACH(file ${_IMPORT_CHECK_FILES_FOR_${target}} )\n"
         "    IF(NOT EXISTS \"${file}\" )\n"
-        "      MESSAGE(FATAL_ERROR \"The imported target \\\"${target}\\\" "
-        "references the file \\\"${file}\\\", but this file does not exist. "
-        "There are multiple possible reasons:\n"
-        " * The file \\\"${file}\\\" has been manually "
-        "deleted, renamed or moved to another location.\n"
-        " * A previous install or uninstall procedure did not complete "
-        " successfully.\n"
-        " * The installation package was faulty, and contained\n"
-        "\\\"${CMAKE_CURRENT_LIST_FILE}\\\"\n"
-        "but not\n"
-        "\\\"${file}\\\"\n"
-        "which must always be installed together.\\n\"\n"
-        "             )\n"
+        "      MESSAGE(FATAL_ERROR \"The imported target \\\"${target}\\\""
+        " references the file\n"
+        "   \\\"${file}\\\"\n"
+        "but this file does not exist.  Possible reasons include:\n"
+        "* The file was deleted, renamed, or moved to another location.\n"
+        "* An install or uninstall procedure did not complete successfully.\n"
+        "* The installation package was faulty and contained\n"
+        "   \\\"${CMAKE_CURRENT_LIST_FILE}\\\"\n"
+        "but not all the files it references.\n"
+        "\")\n"
         "    ENDIF()\n"
         "  ENDFOREACH()\n"
         "  UNSET(_IMPORT_CHECK_FILES_FOR_${target})\n"