Fix import! to throw error when target file/library is not found

[ermijeremy]

Description

Summary

This PR changes the import execution path so missing files/library targets raise a real Prolog exception instead of failing by backtracking to false.

Root Cause

import! previously swallowed all importer failures:

'import!'(Space, File, true) :- catch(importer_helper(Space, File), _, fail).

Implementation Details

The change removes blanket exception swallowing and makes failure explicit.

import! now directly delegates to importer_helper/2:

'import!'(Space, File, true) :- importer_helper(Space, File).

resolution now follows:

1. Build candidate path (Path)
2. Normalize extension with ensure_metta_ext/2 -> PathWithExt
3. If file exists, load via load_metta_file/3
4. Otherwise, throw:

throw(error(existence_error(source_sink, File),
            context('import!', File)))

This yields standard SWI-Prolog error semantics and is catch-compatible.

For valid imports, behavior is unchanged:
path resolution
.metta extension normalization
loading into the target space

Before

 Missing import: silent fail (`false`)

After

Missing import: explicit runtime error (`existence_error(source_sink, ...)`)
`catch` receives a concrete error term and can handle/report it

Validation

`!(catch (import! ...missing...))` returns structured `Error(...)` value as expected.