Less worse error if file mount source doesn't exist

[itowlson]

An ickle wee error message tweak relating to #1022.

Previously:

Error: Failed to load manifest from "spin.toml"

Caused by:
   0: Failed to load Spin app from "spin.toml"
   1: Failed to load component `fstest`
   2: invalid file mount source "assets"
   3: No such file or directory (os error 2)

With this PR:

Error: Failed to load manifest from "spin.toml"

Caused by:
   0: Failed to load Spin app from "spin.toml"
   1: Failed to load component `fstest`
   2: File or directory "assets" does not exist

(Yes, I would like to simplify the stack further, especially those first two lines, but that's a separate issue.)