Do not terminate app if script import has failed

1. Catches exceptions inside the import module, not allowing to reach main() and terminate the application.

2. Cleans function parser history between imports.

3. Shows choosing function window even if only one function is found. This helps user to better understand failure if script contains no separate function defining sample.

Related to #645 (closed)

requested review from @j.wuttke

assigned to @m.svechnikov

changed the description

looks good to me

approved this merge request

mentioned in commit 8019a7a8

merged