Better handling of HTTP errors in EUtils

[mirestrepo]

Expected Behavior

Handle EUtils HTTP errors gracefully

Current Behavior

Some HTTP errors throw rather cryptic errors. For instance if more NCBI's API frequency is exceeded (3 calls/seconds), we get an error

Got an exception of type HTTP.IOExtras.IOError outside of a @test
  IOError(EOFError() during request(https://eutils.ncbi.nlm.nih.gov/entrez/eutils/esummary...

Possible Solution / Implementation

Try/Catch, if error is of the right type, sleep(0.3) try again. We need to investigate if the HTTP error is consistent in this case
Alternatively, we could keep a global representing time since last call?

Steps to Reproduce (for bugs)

If the sleep(0.3) lines are taken out of tests, then they intermittently fail