Fix what is fundamentally a Pyperclip bug on headless Linux systems without Gtk libraries

Author: tleonhardt

CHANGELOG.md

@@ -1,6 +1,7 @@
 ## 0.9.17 (TBD, 2019)
 * Bug Fixes
     * Fixed a bug when using WSL when all Windows paths have been removed from $PATH
+    * Fixed a bug when running a cmd2 application on Linux without Gtk libraries installed
 * Enhancements
     * No longer treating empty text scripts as an error condition
 

cmd2/clipboard.py

@@ -10,8 +10,9 @@
 try:
     # Try getting the contents of the clipboard
     _ = pyperclip.paste()
-except (PyperclipException, FileNotFoundError):
+except (PyperclipException, FileNotFoundError, ValueError):
     # NOTE: FileNotFoundError is for Windows Subsystem for Linux (WSL) when Windows paths are removed from $PATH
+    # NOTE: ValueError is for headless Linux systems without Gtk installed
     can_clip = False
 else:
     can_clip = True