Change fromObj error messages

Currently fromObj doesn't detail what we're trying to read, which can
lead to cryptic messages: "Cannot read Int". The patch changes this
function to annotate the error messages with the key/value we're trying
to convert, by using a new version of fromJResult.

Since the display of the key in tryFromObj is now redundant (it was
already redundant in the 'not found' case), we remove it.

The new version of fromJResult (annotateJResult) simply prepends a
description string to the actual error message.