fix(cli/migrate): tolerate non-UTF-8 in migration

[kornelski]

When I ran cargo tauri migrate I got only:

Error Stream did not contain valid UTF-8

which is a completely unhelpful dead-end.

• I've made stdout/stderr captures tolerate non-UTF-8 text.
• I've added context to migrator's errors that help locate the root cause.
• I've made migrator skip and warn about non-UTF-8 JavaScript files instead of aborting the entire operation.

[amrbashir]

Instead of skipping non-utf8 files, I changed it to replace bytes instead, tested by appending invalid utf8 bytes to a JS file echo -n -e '\x00\x9F\x92\x96' >> src/App.jsx, let me know if I missed anything