internal/stdlib: remove uses of FromHTTPStatus

We should endeavor to eliminate FromHTTPStatus, and just use the
internal error semantics of the derrors package. I'm looking at other
uses, but for now eliminate these trivial uses: just use InvalidArgument
directly.

Change-Id: I55149b43031829953c9ec2a22b20f933f343b933
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/238438
Reviewed-by: Jonathan Amsterdam <jba@google.com>
diff --git a/internal/stdlib/stdlib.go b/internal/stdlib/stdlib.go
index 2dbb18b..e79fe5b 100644
--- a/internal/stdlib/stdlib.go
+++ b/internal/stdlib/stdlib.go
@@ -13,7 +13,6 @@
 	"bytes"
 	"fmt"
 	"io"
-	"net/http"
 	"os"
 	"path"
 	"path/filepath"
@@ -89,7 +88,7 @@
 		return "go1", nil
 	}
 	if !semver.IsValid(version) {
-		return "", derrors.FromHTTPStatus(http.StatusBadRequest, "requested version is not a valid semantic version: %q ", version)
+		return "", fmt.Errorf("%w: requested version is not a valid semantic version: %q ", derrors.InvalidArgument, version)
 	}
 	goVersion := semver.Canonical(version)
 	prerelease := semver.Prerelease(goVersion)
@@ -107,7 +106,7 @@
 		i := finalDigitsIndex(prerelease)
 		if i >= 1 {
 			if prerelease[i-1] != '.' {
-				return "", derrors.FromHTTPStatus(http.StatusBadRequest, "final digits in a prerelease must follow a period")
+				return "", fmt.Errorf("%w: final digits in a prerelease must follow a period", derrors.InvalidArgument)
 			}
 			// Remove the dot.
 			prerelease = prerelease[:i-1] + prerelease[i:]