internal/api: refactor error handling to increase consistency

- Refactored error handling to avoid leaking internal implementation
  details. Database and system errors are masked by falling back to
  standard HTTP statuses but still logging the entire error context.
- User facing error messages can now be specified within the Error
  struct.
- Added helpers in types.go to simplify error construction.
- Updated ServeModuleVersions to explicitly return 404 when no versions
  are found.
- Expanded test coverage in api_test.go to include 404 and 400 edge
  cases.

Change-Id: I89c4be3941126c15df6aefdd21e4bbd2d3b23be1
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/764820
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ethan Lee <ethanalee@google.com>
kokoro-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

1 files changed, 2 insertions, 4 deletions

diff --git a/internal/api/params.go b/internal/api/params.go
index 6858de5e..d0403fe2 100644
--- a/internal/api/params.go
+++ b/internal/api/params.go
@@ -9,8 +9,6 @@ import (
 	"net/url"
 	"reflect"
 	"strconv"
-
-	"golang.org/x/pkgsite/internal/derrors"
 )
 
 // ListParams are common pagination and filtering parameters.
@@ -85,10 +83,10 @@ type VulnParams struct {
 func ParseParams(v url.Values, dst any) error {
 	val := reflect.ValueOf(dst)
 	if val.Kind() != reflect.Pointer || val.Elem().Kind() != reflect.Struct {
-		return fmt.Errorf("%w: dst must be a pointer to a struct", derrors.InvalidArgument)
+		return InternalServerError("dst must be a pointer to a struct")
 	}
 	if err := parseValue(v, val.Elem()); err != nil {
-		return fmt.Errorf("%w: %v", derrors.InvalidArgument, err)
+		return BadRequest("%v", err)
 	}
 	return nil
 }