Fix TypeError from Tags::Reference

What does this MR do and why?

Gitlab::Ci::Config::Yaml::Tags::Reference has been throwing a TypeError when given specific combinations of references and configuration, such as the one below:

a: [1, 2]
c: !reference [a, b]

Because a has no sub-key b, Reference was attempting [1, 2][:b], which resulted in a TypeError because :b cannot be cast to an integer.

This MR updates Reference to catch the TypeError and raise a MissingReferenceError with an appropriate message that will help users fix their configuration.

Resolves: TypeErrorGraphqlController#execute
Part of: #349973 (closed)

Screenshots or screen recordings

The following examples use this CI configuration:

test_job:
  script:
    - echo "test"
    - echo "done

tag_job:
  script:
    - !reference [test_job, script, sub_script]

Before fix:

Screen_Recording_2022-02-16_at_21.59.21

After fix:

Screen_Recording_2022-02-16_at_22.00.14

How to set up and validate locally

1. Go to /-/graphql-explorer

2. Enter the following query:

   query ($content:String!){
     ciConfig(content: $content, projectPath: "flightJS/Flight") {
     errors
       stages {
         nodes {
           name
         }
       }
     }
   }

   where $content is:

   test_job:
     script:
       - echo "test"
       - echo "done
   
   tag_job:
     script:
       - !reference [test_job, script, sub_script]

3. Enter the query, and see that you receive the error below

   

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