Update the exception hierarchy
Update the exception hierarchy

    Everything now inherits from WaterButlerError rather than
    PluginError
    Adds the InvalidParameters exception for generic http rejections

chrisseto/waterbutler,TomBaxter/waterbutler,RCOSDP/waterbutler,icereval/waterbutler,rafaeldelucena/waterbutler,rdhyee/waterbutler,cosenal/waterbutler,CenterForOpenScience/waterbutler,Johnetordoff/waterbutler,felliott/waterbutler,kwierman/waterbutler