Consistent loading state and error handling

[ragingsquirrel3]

Make loading indicator and error handling consistent across pages. The gene page does very rudimentary error handling and the search results page appears to fail silently in certain cases. Both pages use different loading indicators. It would be good to sort this out before we start adding too many more types of pages.

[ragingsquirrel3]

there is a component which I initially designed to give a nice looking, general loading look at https://github.com/alliance-genome/agr/blob/development/src/js_src/components/loadingPage.js, which the search pages renders in place of the content whenever the search is loading. This should be used on any page, whenever the entire page is pending. In the searchReducer, there is a boolean for isPending, so maybe that value could be moved into a more general reducer.