[Fix #12865] require 'bundler' if possible and rescue Bundler::GemfileNotFound

[amomchilov]

Closes #12865

This PR improves upon #12826, which tried to require 'bundler' if it's available, we we reverted after #12846.

The issue wasn't the attempt to require Bundler. It was that we were calling Bundler.default_lockfile, which raises Bundler::GemfileNotFound when you're in a folder with no Gemfile.lock.

I can't test this right now, so this is just "blind coding". Can you try it out, @AmShaegar13

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[AmShaegar13]

Yea, that works. Thanks for taking the time!

[koic]

Thanks!