fix: fail with actionable errors on misconfigured verifier options

[tas50]

The verifier interpolates its config straight into PowerShell. When the config is wrong, the failure used to land somewhere unrelated — or not at all. Each of these is a plausible kitchen.yml typo I could reproduce:

A list option written as a single mapping. install_modules, register_repository, bootstrap.modules and copy_folders are all documented as lists. Leave off the - and Array() turns the mapping into a list of [key, value] pairs. install_modules then generated, silently:

Write-host -NoNewline 'Installing [:Name, "PSScriptAnalyzer"] ...'
Install-Module -Name '[:Name, "PSScriptAnalyzer"]'

register_repository raised TypeError: no implicit conversion of Symbol into Integer from inside the verifier, and copy_folders raised TypeError out of File.join.

A mapping entry with no Name. install_modules raised NoMethodError: undefined method 'gsub' for nil. register_repository and bootstrap.modules emitted an empty ${} into the generated script and failed on the instance, with a PowerShell parse error that says nothing about which entry caused it.

A test_folder that does not exist. Pathname#realpath raised Errno::ENOENT: No such file or directory @ rb_check_realpath_internal - tests, which mentions neither the option nor kitchen.yml. The README already tells people the folder has to exist; now the error does too.

A downloads entry with a blank value. NoMethodError: undefined method 'gsub' for nil.

All of them now raise Kitchen::UserError naming the option at fault and the shape it expects, e.g.:

The verifier's 'install_modules' must be a list, but a single mapping was given. Put a '- ' in front of each entry in kitchen.yml.

The verifier's 'test_folder' is set to 'tests/integration', which does not exist. It is resolved relative to the directory kitchen runs in, so give it a path that exists there or an absolute one.

No valid configuration changes behaviour. spec/kitchen/verifier/pester_config_errors_spec.rb pins each message and, next to it, the shapes that have to keep working — a list of mappings, a list of bare strings, and a Name given as a string key rather than a symbol.

Deliberately untouched: the exit-code handling in #invoke_pester_scriptblock, which is in flight in #109.

Verification

$ bundle exec cookstyle --chefstyle
12 files inspected, no offenses detected

$ bundle exec rake test
164 runs, 341 assertions, 0 failures, 0 errors, 0 skips
Tests Passed: 17, Failed: 0, Skipped: 0