fix: import-url command exits early on first failure when importing multiple URLs

Why we need this:
When importing multiple comma-separated URLs, if one URL fails,
the command immediately exits and remaining URLs are never imported.
This is the same issue that was fixed in the import command in PR #394.

How this will help:
All URLs should be attempted regardless of individual failures,
giving users complete feedback in one run.

Motivation:
Consistent behavior across all import commands (import, import-url).

## Problem
When running:
microcks import-url "https://url1.yaml,https://url2.yaml,https://url3.yaml"

If url1 fails, os.Exit(1) is called immediately inside the loop
and url2, url3 are never imported.

Looking at cmd/importURL.go:
for _, f := range sepSpecificationFiles {
msg, err := mc.DownloadArtifact(f, mainArtifact, secret)
if err != nil {
os.Exit(1) // exits immediately, remaining URLs skipped
}
}

Same bug as #393, fixed in import command via PR #394.

## Fix
- Collect errors instead of exiting immediately
- Attempt all URLs regardless of individual failures
- Exit with code 1 only if any URL failed

Start with cmd/importURL.go and compare the corresponding fix in PR #394 for the import command. Update the import-url loop so every URL is attempted and the command exits with code 1 only when at least one import fails. Verify that later URLs are still processed after an earlier failure.

Written by the indexing model from the issue text.