Deprecate running Crawler.crawl() twice.

[wRAR]

Fixes #1587

[codecov]

Codecov Report

Merging #6040 (da1916c) into master (c1dd549) will decrease coverage by 0.90%.
The diff coverage is 100.00%.

❗ Current head da1916c differs from pull request most recent head b39d2d4. Consider uploading reports for the commit b39d2d4 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6040      +/-   ##
==========================================
- Coverage   88.92%   88.02%   -0.90%     
==========================================
  Files         163      163              
  Lines       11535    11538       +3     
  Branches     1876     1877       +1     
==========================================
- Hits        10257    10156     -101     
- Misses        969     1073     +104     
  Partials      309      309              

Files Changed	Coverage Δ
scrapy/crawler.py	86.50% <100.00%> (-0.81%)	⬇️

... and 15 files with indirect coverage changes

[wRAR]

With 2 it's still not helpful as it points to twisted/internet/defer.py (as inlineCallbacks turns into several frames), but I'm not sure if it's possible to set a correct number here.