Support error serialization

This adds support for error serialization. This is needed for event
streams, which can include error shapes. Consequently, this also
moves errors from `errors.py` into `models.py` to avoid circular
references.

Author: JordonPhillips

codegen/smithy-python-codegen/src/main/java/software/amazon/smithy/python/codegen/CodegenUtils.java

@@ -106,8 +106,8 @@ public static Symbol getPluginSymbol(PythonSettings settings) {
     public static Symbol getServiceError(PythonSettings settings) {
         return Symbol.builder()
                 .name("ServiceError")
-                .namespace(format("%s.errors", settings.moduleName()), ".")
-                .definitionFile(format("./%s/errors.py", settings.moduleName()))
+                .namespace(format("%s.models", settings.moduleName()), ".")
+                .definitionFile(format("./%s/models.py", settings.moduleName()))
                 .build();
     }
 
@@ -123,8 +123,8 @@ public static Symbol getServiceError(PythonSettings settings) {
     public static Symbol getApiError(PythonSettings settings) {
         return Symbol.builder()
                 .name("ApiError")
-                .namespace(format("%s.errors", settings.moduleName()), ".")
-                .definitionFile(format("./%s/errors.py", settings.moduleName()))
+                .namespace(format("%s.models", settings.moduleName()), ".")
+                .definitionFile(format("./%s/models.py", settings.moduleName()))
                 .build();
     }
 
@@ -140,8 +140,8 @@ public static Symbol getApiError(PythonSettings settings) {
     public static Symbol getUnknownApiError(PythonSettings settings) {
         return Symbol.builder()
                 .name("UnknownApiError")
-                .namespace(format("%s.errors", settings.moduleName()), ".")
-                .definitionFile(format("./%s/errors.py", settings.moduleName()))
+                .namespace(format("%s.models", settings.moduleName()), ".")
+                .definitionFile(format("./%s/models.py", settings.moduleName()))
                 .build();
     }
 

codegen/smithy-python-codegen/src/main/java/software/amazon/smithy/python/codegen/StructureGenerator.java

@@ -148,9 +148,12 @@ class $1L($2T):
 
                     ${7C|}
 
+                    ${8C|}
+
                 """, symbol.getName(), apiError, code, fault,
                 writer.consumer(w -> writeClassDocs(true)),
                 writer.consumer(w -> writeProperties()),
+                writer.consumer(w -> generateSerializeMethod()),
                 writer.consumer(w -> generateDeserializeMethod()));
     }
 

codegen/smithy-python-codegen/src/main/java/software/amazon/smithy/python/codegen/SymbolVisitor.java

@@ -330,8 +330,7 @@ private Symbol genericEnum(Shape shape) {
     @Override
     public Symbol structureShape(StructureShape shape) {
         String name = getDefaultShapeName(shape);
-        var file = shape.hasTrait(ErrorTrait.class) ? "errors" : SHAPES_FILE;
-        return createGeneratedSymbolBuilder(shape, name, file).build();
+        return createGeneratedSymbolBuilder(shape, name, SHAPES_FILE).build();
     }
 
     @Override