Validate pipeline node inputs

[proteusvacuum]

We should find a way of returning validation errors to users when building pipelines.

          can we validate this when saving the node as well?

Originally posted by @snopoke in #608 (comment)

[SmittieC]

Here's a swing at it: #821. It will not check that the output from one node is compatable with the input of the next, but the node's config params have input validation (or checking rather)