Return reduce overflow errors to the client

[davisp]

This changes the reduce overflow error to return an error to the client
rather than blowing up the view build. This allows views that have a
single bad reduce to build while not crushing the server's RAM usage.

Testing recommendations

make javascript suites=view_errors

Checklist

• Code is written and works correctly;
• Changes are covered by tests;
• Documentation reflects the changes;

[tonysun83]

I noticed in fabric_util we have an error condition for this error, and also in view_errors we're testing for this situation. Was this error not propagated correctly before and thus the view builds blew up?

[davisp]

@tonysun83 My bad! Forgot to push after I found the view_errors.js test failure. I hadn't noticed the fabric_util clause but I've removed it since it'd be dead code now.

[tonysun83]

+1

[janl]

rebased on latest master, can be merged after travis