Don't swallow control-C (#588)

* Don't swallow control-C

* Raise NimbleError instead of quitting

* Import NimbleError from common rather than version

src/nimblepkg/cli.nim

@@ -13,6 +13,7 @@
 #   - Normal for MediumPriority.
 
 import logging, terminal, sets, strutils, os
+import ./common
 
 when defined(windows):
   import winlean
@@ -218,6 +219,9 @@ proc promptListInteractive(question: string, args: openarray[string]): string =
       of '\r':
         selected = true
         break
+      of '\3':
+        showCursor(stdout)
+        raise newException(NimbleError, "Keyboard interrupt")
       else: discard
 
   # Erase all lines of the selection