repl: do not crash when tab-completing import errors

File not found while importing causes a SysError, not an EvalError,
which is not currently caught by the tab-completion handler. Ignoring
all SysErrors might seem "dangerous" but this is the tab-completion
handler, any exception being bubbled up from there causes unexpected
behavior (causes the whole repl to exit).

Fixes #340.

Change-Id: I643048a47935e77f582decc539d9e51bdb96c890

src/libcmd/repl.cc

@@ -375,6 +375,9 @@ StringSet NixRepl::completePrefix(const std::string & prefix)
             // Quietly ignore evaluation errors.
         } catch (BadURL & e) {
             // Quietly ignore BadURL flake-related errors.
+        } catch (SysError & e) {
+            // Quietly ignore system errors which can for example be raised by
+            // a non-existent file being `import`-ed.
         }
     }