Web Files error UX

[SpComb]

The Web UI needs some error UX for Files/FileNode and ConfigView operations.

Originally posted by @SpComb in #50 (comment)

[SpComb]

#107 + #109 should fix the Config UX

Files error UX I forget what practical cases there were... 7323760 adds a global error -> alert() handler which takes care of the very basics.