Add a check for Unicode support.

This makes poezio refuse to run if the environment is known broken.

poezio/__main__.py

@@ -1,7 +1,7 @@
 def run():
-    from poezio.poezio import main, test_curses, test_env
+    from poezio.poezio import main, test_curses, test_env, test_unicode
 
-    if not test_curses() or not test_env():
+    if not test_curses() or not test_env() or not test_unicode():
         import sys
         sys.exit(1)
     else:

poezio/poezio.py

@@ -53,6 +53,23 @@ def test_env():
     return True
 
 
+def test_unicode():
+    import poopt
+    try:
+        poopt.wcswidth('✔')
+    except UnicodeError:
+        print("""\
+ERROR: The current system is misconfigured for Unicode.
+
+Check your locale setup, especially the $LANG environment variable and \
+whether it matches a locale built on your system.  Also check that it is a \
+.UTF-8 locale, and not using some legacy encoding.
+
+Poezio is unable to display characters properly, so it will now exit.""")
+        return False
+    return True
+
+
 def main():
     """
     Entry point.