Added better error logging for API limit threshold

[PatrickAlphaC]

• closes #xxxx
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• passes black --check pandas_datareader
• added entry to docs/source/whatsnew/vLATEST.txt

Now when the API rate limit is hit instead of the error being:

--> 81                 raise RemoteDataError()
     82         df = df[sorted(df.columns)]
     83         df.columns = [id[3:] for id in df.columns]

RemoteDataError: 

It will be:

pandas_datareader._utils.RemoteDataError: {'Note': 'Thank you for using Alpha Vantage! Our standard API call frequency is 5 calls per minute and 500 calls per day. Please visit https://www.alphavantage.co/premium/ if you would like to target a higher API call frequency.'}

[codecov-io]

Codecov Report

Merging #745 into master will decrease coverage by 13.58%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master     #745       +/-   ##
===========================================
- Coverage   83.74%   70.15%   -13.59%     
===========================================
  Files          62       62               
  Lines        2768     2768               
  Branches      305      305               
===========================================
- Hits         2318     1942      -376     
- Misses        375      749      +374     
- Partials       75       77        +2

Impacted Files	Coverage Δ
pandas_datareader/av/forex.py	22.72% <0%> (-72.73%)	⬇️
pandas_datareader/av/quotes.py	26.47% <0%> (-64.71%)	⬇️
pandas_datareader/quandl.py	29.26% <0%> (-63.42%)	⬇️
pandas_datareader/av/time_series.py	32.43% <0%> (-62.17%)	⬇️
pandas_datareader/enigma.py	23.63% <0%> (-60%)	⬇️
pandas_datareader/tiingo.py	38.29% <0%> (-57.45%)	⬇️
pandas_datareader/tests/test_quandl.py	42.59% <0%> (-57.41%)	⬇️
pandas_datareader/tests/av/test_av_time_series.py	28.4% <0%> (-51.14%)	⬇️
pandas_datareader/tests/av/test_av_forex.py	50% <0%> (-50%)	⬇️
pandas_datareader/tests/av/test_av_quotes.py	50% <0%> (-46.88%)	⬇️
... and 8 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 57766a5...b345e54. Read the comment docs.

[addisonlynch]

@PatrickAlphaVantage thanks!

[PatrickAlphaC]

So it looks like the builds were failing before I made this PR, is master broken at the moment? Or did I do something silly.

[addisonlynch]

@PatrickAlphaVantage yeah failing for different reasons

[bashtage]

Would be best to reformat out with some text that would help user understand the error message.

[bashtage]

Thanks.