Derive "param_hint" in errors from param itself

Rather than trying to apply the same logic to all param types, trying to
extract a meaningful value from Parameter.opts even for Arguments, make
this a function computed against the Parameter itself. Then, when it's
time to specialize it to use the metavar for Arguments, we just need to
override the function.
Also unify a small amount of logic being shared among exception classes.
Fully backwards compatible, except for the change to error text on
Arguments, which is the intended effect.

Fixes pallets#704, pallets#598

click/core.py

@@ -1431,6 +1431,13 @@ def get_help_record(self, ctx):
     def get_usage_pieces(self, ctx):
         return []
 
+    def get_error_hint(self, ctx):
+        """Get a stringified version of the param for use in error messages to
+        indicate which param caused the error.
+        """
+        hint_list = self.opts or [self.human_readable_name]
+        return ' / '.join('"%s"' % x for x in hint_list)
+
 
 class Option(Parameter):
     """Options are usually optional values on the command line and
@@ -1755,6 +1762,9 @@ def _parse_decls(self, decls, expose_value):
     def get_usage_pieces(self, ctx):
         return [self.make_metavar()]
 
+    def get_error_hint(self, ctx):
+        return self.make_metavar()
+
     def add_to_parser(self, parser, ctx):
         parser.add_argument(dest=self.name, nargs=self.nargs,
                             obj=self)

click/exceptions.py

@@ -2,6 +2,12 @@
 from .utils import echo
 
 
+def _join_param_hints(param_hint):
+    if isinstance(param_hint, (tuple, list)):
+        return ' / '.join('"%s"' % x for x in param_hint)
+    return param_hint
+
+
 class ClickException(Exception):
     """An exception that Click can handle and show to the user."""
 
@@ -92,11 +98,11 @@ def format_message(self):
         if self.param_hint is not None:
             param_hint = self.param_hint
         elif self.param is not None:
-            param_hint = self.param.opts or [self.param.human_readable_name]
+            param_hint = self.param.get_error_hint(self.ctx)
         else:
             return 'Invalid value: %s' % self.message
-        if isinstance(param_hint, (tuple, list)):
-            param_hint = ' / '.join('"%s"' % x for x in param_hint)
+        param_hint = _join_param_hints(param_hint)
+
         return 'Invalid value for %s: %s' % (param_hint, self.message)
 
 
@@ -121,11 +127,10 @@ def format_message(self):
         if self.param_hint is not None:
             param_hint = self.param_hint
         elif self.param is not None:
-            param_hint = self.param.opts or [self.param.human_readable_name]
+            param_hint = self.param.get_error_hint(self.ctx)
         else:
             param_hint = None
-        if isinstance(param_hint, (tuple, list)):
-            param_hint = ' / '.join('"%s"' % x for x in param_hint)
+        param_hint = _join_param_hints(param_hint)
 
         param_type = self.param_type
         if param_type is None and self.param is not None: