assets: add an error to ob run when file doesn't exist

[ali-abrar]

Before this PR, and since #831, calling static with a nonexistent file would not result in an error or a warning in ob run, but would fail at compile-time when using ob deploy or nix-build.

This PR adds errors like the following to static in ob run:

frontend/src/Frontend.hs:45:32-55: error:
    • Static file obeliskasdf.jpg was not found in static.out
    • In the untyped splice: $(static "obeliskasdf.jpg")
   |
45 |       elAttr "img" ("src" =: $(static "obeliskasdf.jpg"))
blank
   |                                ^^^^^^^^^^^^^^^^^^^^^^^^

Notes:

• Since nix-derived assets are built asynchronously, this error might be reported because the nix-build of the static assets isn't ready yet. The user would have trigger a ghci reload (by saving a file) to get rid of the error.
• The TH error will only be reported when the haskell code is being reloaded. Changing the files in static/ currently does not trigger a ghci reload and won't be noticed.
• Related PR: Don't wait for nix show-derivation when deciding whether to provide UI feedback #835

I have:

• Based work on latest develop branch
• Followed the contribution guide
• Looked for lint in my changes with hlint . (lint found code you did not write can be left alone)
• Run the test suite: $(nix-build -A selftest --no-out-link)
• Updated the changelog
• (Optional) Run CI tests locally: nix-build release.nix -A build.x86_64-linux --no-out-link (or x86_64-darwin on macOS)