ENH: Improve error when ARMA endog is not 1d

[bashtage]

Improve explanation when passing an array that is not 1d or 1 column, 2d

closes #2575

• closes ARIMA: Check if input dataframe has one column #2575
• tests added / passed.
• code/documentation is well formatted.
• properly formatted commit message. See
  NumPy's guide.

[coveralls]

Coverage increased (+0.01%) to 84.556% when pulling e6e6199 on bashtage:arma-1d-check into 6453c81 on statsmodels:master.

[codecov]

Codecov Report

Merging #5640 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #5640      +/-   ##
==========================================
+ Coverage   82.02%   82.03%   +<.01%     
==========================================
  Files         587      587              
  Lines       93435    93451      +16     
  Branches    10374    10375       +1     
==========================================
+ Hits        76642    76658      +16     
  Misses      14392    14392              
  Partials     2401     2401

Impacted Files	Coverage Δ
statsmodels/tsa/arima_model.py	86.91% <100%> (+0.04%)	⬆️
statsmodels/tsa/tests/test_arima.py	99.5% <100%> (ø)	⬆️
statsmodels/tsa/tests/test_holtwinters.py	100% <0%> (ø)	⬆️
statsmodels/regression/tests/test_lme.py	99.46% <0%> (ø)	⬆️

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