Improve specs for Array#fetch_values

[herwinw]

Reference: #1265

This one had a pretty extensive spec that was included with the implementation in MRI. I added two commits:

• Add explicit error messages in the should raise_error checks, mostly because this makes it much easier for alternative Ruby implementations to match the error message.
• Add an explicit spec for a Range argument. This got mentioned on https://rubyreferences.github.io/rubychanges/3.4.html#arrayfetch_values as an ongoing issue (Ruby tracker: https://bugs.ruby-lang.org/issues/20953, the link is missing on the rubychanges site, I made a PR to fix that). I am not sure if this should be added now, or that we should just wait for the next Ruby release to include the behaviour, especially since https://github.com/ruby/ruby/pull/12565/files#diff-a66359f06e4d70da38c973522b616e02a867d47cbe1949102fc575634b402510 already contains the updates to the spec.

[andrykonchin]

Thank you!