Fix crash when when no platform specific matches exist and show a proper error

[deivid-rodriguez]

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

When a gem only has platform specific variants and none of them matches the current platform, the resolver would incorrectly allow this case, and we would get an error later down the line since that specific gem can't be materialized.

See #5088 (comment).

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

My fix is to properly detect this case and don't consider any candidates suffering from this problem, resulting in a proper resolution error rather than a crash.

Also took the chance to improve the error message in this case.

So, now, instead of the previous output with a runtime crash and the bug report template, we get an informative message, like this

$ bundle install
Fetching gem metadata from https://rubygems.org/........
Resolving dependencies...
Bundler could not find compatible versions for gem "sorbet-static":
  In Gemfile:
    sorbet (= 0.5.6433) was resolved to 0.5.6433, which depends on
      sorbet-static (= 0.5.6433)

Could not find gem 'sorbet-static (= 0.5.6433) arm64-darwin-21', which is required by gem 'sorbet (= 0.5.6433)', in rubygems repository https://rubygems.org/ or installed locally.

The source contains the following gems matching 'sorbet-static (= 0.5.6433)':
  * sorbet-static-0.5.6433-java
  * sorbet-static-0.5.6433-universal-darwin-14
  * sorbet-static-0.5.6433-universal-darwin-15
  * sorbet-static-0.5.6433-universal-darwin-16
  * sorbet-static-0.5.6433-universal-darwin-17
  * sorbet-static-0.5.6433-universal-darwin-18
  * sorbet-static-0.5.6433-universal-darwin-19
  * sorbet-static-0.5.6433-universal-darwin-20
  * sorbet-static-0.5.6433-x86_64-linux

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