repl: do not crash when tab-completing import errors

File not found while importing is not currently caught by the tab-completion handler.

Original bug report: https://git.lix.systems/lix-project/lix/issues/340
Fix has been adapted from https://gerrit.lix.systems/c/lix/+/1189

Example crash:

$ cat /tmp/foo.nix
{
  someImport = import ./this_file_does_not_exist;
}
$ ./src/nix/nix repl --file /tmp/foo.nix
warning: unknown experimental feature 'repl-flake'
Nix 2.23.0pre20240517_dirty
Type :? for help.
Loading installable ''...
Added 1 variables.
nix-repl> someImport.<TAB>

src/libcmd/repl.cc

@@ -304,6 +304,8 @@ StringSet NixRepl::completePrefix(const std::string & prefix)
             // Quietly ignore evaluation errors.
         } catch (BadURL & e) {
             // Quietly ignore BadURL flake-related errors.
+        } catch (FileNotFound & e) {
+            // Quietly ignore non-existent file beeing `import`-ed.
         }
     }