refactor: standardize error message formatting

Makes all error messages lowercase and adds attribute context to type
errors for consistency with Ruby conventions and improved debugging.

Changes:
- Lowercase error messages: "cannot specify..." instead of "Cannot specify..."
- Add attribute name to type errors: "cannot specify X as type for :attr"
- Catch and re-raise ArgumentError in Builder#coercions to add context
- Update tests to match new error message format

Closes #16

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: hakanensari

CHANGELOG.md

@@ -21,6 +21,11 @@ This project adheres to [Semantic Versioning](http://semver.org/).
   User.parse(id: "123")                              # ✓ valid (name is optional)
   ```
 
+### Changed
+
+- Standardize error messages to use lowercase for consistency with Ruby conventions
+- Improve type error messages to include attribute name for easier debugging
+
 ## [4.1.0] - 2025-10-09
 
 ### Added

lib/structure/builder.rb

@@ -46,7 +46,7 @@ def attribute(name, type = nil, from: nil, default: nil, null: true, &block)
       @non_nullable.add(name) unless null
 
       if type && block
-        raise ArgumentError, "Cannot specify both type and block for :#{name}"
+        raise ArgumentError, "cannot specify both type and block for :#{name}"
       else
         types[name] = type || block
       end
@@ -102,7 +102,12 @@ def non_nullable = @non_nullable.to_a
 
     # @api private
     def coercions(context = nil)
-      @types.transform_values { |type| Types.coerce(type, context) }
+      @types.to_h do |attr, type|
+        coercion = Types.coerce(type, context)
+        [attr, coercion]
+      rescue ArgumentError => e
+        raise ArgumentError, "#{e.message} for :#{attr}"
+      end
     end
 
     # @api private

lib/structure/types.rb

@@ -60,7 +60,7 @@ def coerce(type, context = nil)
         when String
           lazy_class(type, context)
         else
-          raise ArgumentError, "Cannot specify #{type.inspect} as type"
+          raise ArgumentError, "cannot specify #{type.inspect} as type"
         end
       end
 

test/test_core_structure.rb

@@ -85,7 +85,7 @@ def test_attribute_with_both_type_and_block_raises_error
       end
     end
 
-    assert_match(/Cannot specify both type and block/, error.message)
+    assert_match(/cannot specify both type and block/, error.message)
   end
 
   def test_attribute_with_default_value

test/test_type_coercions.rb

@@ -152,10 +152,12 @@ def test_empty_hash_type_raises_error
   def test_invalid_object_type_raises_error
     invalid_object = Object.new
 
-    assert_raises(ArgumentError) do
+    error = assert_raises(ArgumentError) do
       Structure.new do
         attribute(:value, invalid_object)
       end
     end
+
+    assert_match(/cannot specify.*as type for :value/, error.message)
   end
 end