Fix "NameError: uninitialized constant Exceptions"

Zeitwerk operates on files in the autoload paths. Since the
entire "app" directory is autoloaded, all files inside need to
play by Zeitwerk's rules. Zeitwerk expects that a file named
"exceptions.rb" would define a constant named "Exceptions".

The exceptions file doesn't follow Zeitwerk's conventions.
The easiest way to solve this is to move the execptions out of
app/lib and into lib, where `require_relative` doesn't have
Zeitwerk taken into account.

lib/exceptions.rb

@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+module GabSocial
+  class Error < StandardError; end
+  class NotPermittedError < Error; end
+  class ValidationError < Error; end
+  class HostValidationError < ValidationError; end
+  class LengthValidationError < ValidationError; end
+  class DimensionsValidationError < ValidationError; end
+  class RaceConditionError < Error; end
+
+  class UnexpectedResponseError < Error
+    def initialize(response = nil)
+      if response.respond_to? :uri
+        super("#{response.uri} returned code #{response.code}")
+      else
+        super
+      end
+    end
+  end
+end