tap-(un)pin/untap: better error message

Closes Homebrew/homebrew#49904.

Signed-off-by: Xu Cheng <xucheng@me.com>

Library/Homebrew/cmd/tap-pin.rb

@@ -4,7 +4,7 @@ module Homebrew
   def tap_pin
     ARGV.named.each do |name|
       tap = Tap.fetch(name)
-      raise "#{tap} is not allowed" if tap.core_tap?
+      raise "pinning #{tap} is not allowed" if tap.core_tap?
       tap.pin
       ohai "Pinned #{tap}"
     end

Library/Homebrew/cmd/tap-unpin.rb

@@ -4,7 +4,7 @@ module Homebrew
   def tap_unpin
     ARGV.named.each do |name|
       tap = Tap.fetch(name)
-      raise "#{tap} is not allowed" if tap.core_tap?
+      raise "unpinning #{tap} is not allowed" if tap.core_tap?
       tap.unpin
       ohai "Unpinned #{tap}"
     end

Library/Homebrew/cmd/untap.rb

@@ -6,7 +6,7 @@ module Homebrew
 
     ARGV.named.each do |tapname|
       tap = Tap.fetch(tapname)
-      raise "#{tap} is not allowed" if tap.core_tap?
+      raise "untapping #{tap} is not allowed" if tap.core_tap?
       tap.uninstall
     end
   end