add custom error in case user forgets to define name with let

[SteveRedka]

If you declare a name parameter, but forget to define it within it or let group, RSpec will raise undescriptive error. Two things can happen:

1. Usually it will raise NoMethodError. Which is mostly true, but it's very generic and doesn't say anything about the place where this error happened and how to fix it.

undefined method `blog_id' for #<RSpec::ExampleGroups::ApiTrackers::ApiTrackersIdd::Get::Successful:0x000055e89bf1dab8>

2. Some variables like id can raise WrongScopeError

`id` is not available from within an example (e.g. an `it` block) or from constructs that run in the scope of an example (e.g. `before`, `let`, etc). It is only available on an example group (e.g. a `describe` or `context` block).

The text of this error is just wrong and sends user on wild goose chase.

This pull request adds custom error to remind user to define the missing variable.

[jtannas]

I like it! I've definiely chased that wild goose a couple times and this would've saved a ton of time 🦢
This is also applicable to query parameters with required: true if you feel like extending the PR a bit. If not we can just merge as-is.