catch exceptions in splitCompletionLine (#77)

We currently return `@[]` in `splitCompletionLine` when there is
unexpected or unsupported input via environment variables, but let
exceptions during parsing propagate to the caller. Catching those
exceptions allows the caller to use same behaviour regardless of
the nature of unexpected input (either through env, or parsing).
Currently, callers don't seem to be aware of the exceptions, so
going with the behaviour used for environment errors of returning `@[]`.

confutils/shell_completion.nim

@@ -133,12 +133,15 @@ proc splitCompletionLine*(): seq[string] =
 
   # Split the resulting string
   var l: ShellLexer
-  l.open(strm)
-  while true:
-    let token = l.getTok()
-    if token.isNone():
-      break
-    result.add(token.get())
+  try:
+    l.open(strm)
+    while true:
+      let token = l.getTok()
+      if token.isNone():
+        break
+      result.add(token.get())
+  except IOError, OSError:
+    return @[]
 
 proc shellQuote*(word: string): string =
   if len(word) == 0: