BUG #16265: psql shows (unfolded) relation name in quotes

The following bug has been logged on the website:

Bug reference:      16265
Logged by:          Jose Luis Tallon
Email address:      jltallon@adv-solutions.net
PostgreSQL version: 12.1
Operating system:   Linux
Description:

When dealing with schemas containing non-all-lowercase names, psql error
messages can be confusing.

Reproducer below:
-------------------
test=> CREATE TABLE "Schedule";
CREATE TABLE
test=> \dt "Schedule" 
          List of relations
 Schema |   Name   | Type  |  Owner   
--------+----------+-------+----------
 public | Schedule | table | jltallon
(1 row)
test=> \d+ Schedule
Did not find any relation named "Schedule".
-------------------

EXPECTED:
-------------------
test=> \d+ Schedule
Did not find any relation named "schedule".
-------------------

Please note that the only difference lies in case-folding of the unquoted
relation name: psql copies the relation name verbatim to the output message,
whereas it should case-fold it according to standard Postgres rules.