message: Properly handle SPVC errors.
  Also removes a MaybeError from a function that can't return errors.
diff: @@ -123,19 +123,16 @@ namespace dawn_native {
  UNREACHABLE();
  }
  - ResultOrError<SingleShaderStage> ToSingleShaderStage(
  - shaderc_spvc_execution_model execution_model) {
  + SingleShaderStage ToSingleShaderStage(shaderc_spvc_execution_model execution_model) {
  switch (execution_model) {
  case shaderc_spvc_execution_model_vertex:
  - return {SingleShaderStage::Vertex};
  + return SingleShaderStage::Vertex;
  case shaderc_spvc_execution_model_fragment:
  - return {SingleShaderStage::Fragment};
  + return SingleShaderStage::Fragment;
  case shaderc_spvc_execution_model_glcompute:
  - return {SingleShaderStage::Compute};
  + return SingleShaderStage::Compute;
  default:
  UNREACHABLE();
  - return DAWN_VALIDATION_ERROR(
  - "Attempted to convert invalid spvc execution model to SingleShaderStage");
  }
  }
  @@ -360,8 +357,7 @@ namespace dawn_native {
  shaderc_spvc_execution_model execution_model;
  DAWN_TRY(CheckSpvcSuccess(mSpvcContext.GetExecutionModel(&execution_model),
  "Unable to get execution model for shader."));
  -
  - DAWN_TRY_ASSIGN(mExecutionModel, ToSingleShaderStage(execution_model));
  + mExecutionModel = ToSingleShaderStage(execution_model);
  size_t push_constant_buffers_count;
  DAWN_TRY(
  @@ -804,7 +800,7 @@ namespace dawn_native {
  MaybeError ShaderModuleBase::CheckSpvcSuccess(shaderc_spvc_status status,
  const char* error_msg) {
  if (status != shaderc_spvc_status_success) {
  - DAWN_VALIDATION_ERROR(error_msg);
  + return DAWN_VALIDATION_ERROR(error_msg);
  }
  return {};
  }
  