Improve error message for `box::use('foo')`

[klmr]

Please describe your feature request

The current error messages for the calls

box::use('foo')

and

box::use(./'foo')

are confusing:

Error in box::use("foo") : unexpected token in “foo” (inside “parse_spec_impl(expr)”)

and

Error in box::use(./"foo") : expected module name or attach list, got “foo” (inside “parse_mod(expr)”)

Improve them:

• [x] Remove the unnecessary internal call details (‘(inside “…”)’)
• [x] Be more specific about the token, don’t use “unexpected token” or the raw token value; instead, use something along the lines ‘unexpected character string literal “foo”’.