Be more informative when the signature has an invalid type

lib/nostr/errors/invalid_signature_type_error.rb

@@ -11,6 +11,6 @@ module Nostr
     # @example
     #   InvalidSignatureTypeError.new
     #
-    def initialize = super('Invalid signature type')
+    def initialize = super('Invalid signature type. It must be a string with lowercase hexadecimal characters.')
   end
 end

spec/nostr/errors/invalid_signature_type_error_spec.rb

@@ -7,7 +7,7 @@ RSpec.describe Nostr::InvalidSignatureTypeError do
     let(:error) { described_class.new }
 
     it 'builds a useful error message' do
-      expect(error.message).to eq('Invalid signature type')
+      expect(error.message).to eq('Invalid signature type. It must be a string with lowercase hexadecimal characters.')
     end
   end
 end

spec/nostr/signature_spec.rb

@@ -15,7 +15,7 @@ RSpec.describe Nostr::Signature do
       it 'raises an InvalidSignatureTypeError' do
         expect { described_class.new(1234) }.to raise_error(
           Nostr::InvalidSignatureTypeError,
-          'Invalid signature type'
+          'Invalid signature type. It must be a string with lowercase hexadecimal characters.'
         )
       end
     end