Update UsageError message

[noviluni]

This message was added 10 years ago (Scrapy 0.25.1), and I think that we can update it like this, as the feature of running more than one spider was deprecated a long time ago.

However, as this appears when we write more than one arg, it could be also a good idea to reformulate the whole message to give a hint like "did you forget to add -a / -s , etc". Or something similar.

Let me know what you think. :)

[codecov]

Codecov Report

Merging #5063 (739d0d5) into master (72e8cea) will increase coverage by 0.19%.
The diff coverage is 0.00%.

❗ Current head 739d0d5 differs from pull request most recent head 64d4ae1. Consider uploading reports for the commit 64d4ae1 to get more accurate results

@@            Coverage Diff             @@
##           master    #5063      +/-   ##
==========================================
+ Coverage   87.63%   87.82%   +0.19%     
==========================================
  Files         162      162              
  Lines       10301    10301              
  Branches     1501     1501              
==========================================
+ Hits         9027     9047      +20     
+ Misses       1001      982      -19     
+ Partials      273      272       -1     

Impacted Files	Coverage Δ
scrapy/commands/crawl.py	60.00% <0.00%> (ø)
scrapy/core/downloader/__init__.py	92.48% <0.00%> (+1.50%)	⬆️
scrapy/robotstxt.py	97.53% <0.00%> (+22.22%)	⬆️

[noviluni]

I see that it could be a good idea to add a test too.

[Gallaecio]

I see that it could be a good idea to add a test too.

Do you want to give it a try as part of this pull request, or shall we merge and create a separate ticket for it?

[kmike]

Let's handle tests separately, and get the improvement in. These tests would be unrelated to the change anyways - you can't write a test which checks that the readability improved :)