Avoid error reporting relative path when validating frozen

[yaauie]

When invoking Bundler::Runtiume::setup on a frozen bundle, and there is no
relative path from PWD to our default lockfile, our attempt to pre-generate a
helpful message results in an ArgumentError.

This is more likely to occur on Windows, for example if PWD is on the C:
volume and our project is on the D: volume, our application entry-point is
invoked using an absolute path, and our application invokes Bundler::setup.

What was the end-user or developer problem that led to this PR?

A customer invoking Logstash 7.15.x using an absolute path D:\logstash-7.15.0\bin\logstash from a shell whose PWD was on the C:\ volume resulted in an ArgumentError propagating up from Bundler::Definition#ensure_equivalent_gemfile_and_lockfile. Logstash invokes Bundler directly in code with :frozen attribute set to true.

What is your fix for the problem, implemented in this PR?

Stdlib's Pathname#relative_path_from raises an ArgumentError if it cannot give a relative path.

Since Bundler's usage doesn't actually require a relative path but is only meant to provide helpful guidance to the user should our invocation eventually fail, it makes sense to rescue the ArgumentError and simply use the absolute path when a relative path can't be built.

I would appreciate guidance on how to write a test for this. Bundler::Definition#ensure_equivalent_gemfile_and_lockfile does not appear to be covered at all in unit tests, and in any case there are likely to be OS-specific instructions for what constitutes a different base (on my POSIX machine running jruby, I cannot get Pathname#relative_path_from to produce the same ArgumentError).

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags

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[deivid-rodriguez]

Normally bundler uses end to end specs. I would grep specs for this specific error message and use the result as a base to reproduce this particular realworld situation (or a simplified version of it).

The changes look good to me, by the way!

[deivid-rodriguez]

Actually one of our Windows specs just reproduced this error. See #6989.

So I went ahead and edited this PR so that this similar error you were running into is also prevented.

[deivid-rodriguez]

Thanks!