Make OpenEyeToolkitWrapper failures more verbose

[j-wags]

Is your feature request related to a problem? Please describe.
Some OpenEye toolkits aren't very forthcoming with their error messages, which makes problems hard to debug (eg #492)

Describe the solution you'd like
Use the code like the following to make the OETK error reach the user:

errfs = oechem.oeosstream()
oechem.OEThrow.SetOutputStream(errfs)
oechem.OEThrow.Clear()
quacpac_status = oequacpac.OEAssignCharges(oemol, oequacpac.OEAM1BCCELF10Charges())
oechem.OEThrow.SetOutputStream(oechem.oeerr)#restoring to original state
print(errfs.str().decode("UTF-8"))

It would be best to append the error message text to a OFFTK Exception, so that the OETK error will play nicely with the ToolkitRegistry's try/except structure