Improve error message when plural macro not passed a block

[odow]

Closes #3254

[codecov]

Codecov Report

Base: 98.09% // Head: 98.09% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (9d82a6c) compared to base (6329762).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3255   +/-   ##
=======================================
  Coverage   98.09%   98.09%           
=======================================
  Files          34       34           
  Lines        4663     4664    +1     
=======================================
+ Hits         4574     4575    +1     
  Misses         89       89           

Impacted Files	Coverage Δ
src/macros.jl	98.57% <100.00%> (+<0.01%)	⬆️

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