FIX raise proper error message when only categorical passed to SMOTE-NC

[glemaitre]

closes #562

Add a proper error message when only categorical features are provided to SMOTE-NC

[codecov]

Codecov Report

Merging #720 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #720   +/-   ##
=======================================
  Coverage   96.47%   96.48%           
=======================================
  Files          82       82           
  Lines        5026     5035    +9     
=======================================
+ Hits         4849     4858    +9     
  Misses        177      177           

Impacted Files	Coverage Δ
imblearn/over_sampling/_smote.py	97.23% <100.00%> (+0.01%)	⬆️
imblearn/over_sampling/tests/test_smote_nc.py	100.00% <100.00%> (ø)

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[lgtm-com]

This pull request introduces 1 alert when merging ecc33d0 into 91b99ce - view on LGTM.com

new alerts:

• 1 for Unused import