Reset $XONSH_COLOR_STYLE on bad value

Without doing this, an error message is printed on every iteration
of the command loop.

Perhaps it might be better to reset to the previous value if it exists,
else 'default'?

xonsh/pyghooks.py

@@ -321,6 +321,7 @@ class XonshStyle(Style):
             warn('Could not find style {0!r}, using default'.format(value),
                  RuntimeWarning)
             value = 'default'
+            builtins.__xonsh_env__['XONSH_COLOR_STYLE'] = value
         cmap = STYLES[value]
         try:
             self._smap = get_style_by_name(value)().styles.copy()