Fix an issue where opt crashed when loading a non-existent plugin library

[pzzp]

Summary

The function runPassPipeline forgets to consume the error of load plugin library but just print an message, so it ends up with an unexpected assert failure when LLVM_ENABLE_ABI_BREAKING_CHECKS enabled.

llvm/tools/opt/NewPMDriver.cpp

     if (!PassPlugin) {
       errs() << "Failed to load passes from '" << PluginFN
              << "'. Request ignored.\n";
+      consumeError(PassPlugin.takeError());
       continue;
     }