Make errors with scenarios' name more clear

[joanlopez]

What?

It slightly modifies the errors related with scenarios' name (like empty or prohibited characters).

Why?

Because, as stated in #3601, the current error messages are misleading.

I might be wrong, but in my opinion, I understand that every object under the scenarios key is a scenario definition, thus the key is the name of the scenario, not the name of the executor (type). Also, that's what can be guessed from our docs:

export const options = {
  scenarios: {
    arbitrary_scenario_name: {
      //Name of executor
      executor: 'ramping-vus',
      // more configuration here
    },
  },
};

So, arbitrary_scenario_name is the scenario name, and ramping-vus is the executor, it's name/type.

Checklist

• I have performed a self-review of my code.
• I have added tests for my changes.
• I have run linter locally (make lint) and all checks pass.
• I have run tests locally (make tests) and all tests pass.
• I have commented on my code, particularly in hard-to-understand areas.

Related PR(s)/Issue(s)

Closes #3601

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.54%. Comparing base (f27cca5) to head (3c45e26).

❗ Current head 3c45e26 differs from pull request most recent head 02aca9f. Consider uploading reports for the commit 02aca9f to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3655      +/-   ##
==========================================
- Coverage   73.58%   73.54%   -0.05%     
==========================================
  Files         277      275       -2     
  Lines       20247    20242       -5     
==========================================
- Hits        14898    14886      -12     
- Misses       4401     4404       +3     
- Partials      948      952       +4     

Flag	Coverage Δ
macos	73.52% <100.00%> (+0.03%)	⬆️
ubuntu	73.52% <100.00%> (+0.02%)	⬆️
windows	?

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