fixed #6556 by making the error more informative

[venom1204]

In this i have made the error popping out to ber more informative by two things

1. Which column(s) are missing.
2. Which data.table is missing the column(s).

in this i have also modified the test examples in the /home/vinni/data.table/inst/tests
named
Test 1601.2, 1601.3, 1601.4
Test 1880.1 1880.2
Test 1962.016 1962.018, 1962.019, 1962.021
Test 2145.2
Test 2230.7

[codecov]

Codecov Report

Attention: Patch coverage is 97.36842% with 1 line in your changes missing coverage. Please review.

Project coverage is 98.61%. Comparing base (3b2812b) to head (ee5ac99).

Files with missing lines	Patch %	Lines
R/merge.R	97.36%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6690      +/-   ##
==========================================
- Coverage   98.61%   98.61%   -0.01%     
==========================================
  Files          79       79              
  Lines       14559    14557       -2     
==========================================
- Hits        14358    14355       -3     
- Misses        201      202       +1     

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