store: Return an error from DeleteBlueprint and DeleteBlueprintFromWorkspace

If an unknown blueprint or workspace is deleted it will now return an
error.

Also fixes the blueprints DELETE handlers to return the correct error to
the client. Includes a new test.

internal/weldr/api_test.go

@@ -366,6 +366,7 @@ func TestBlueprintsDelete(t *testing.T) {
 		ExpectedJSON   string
 	}{
 		{"DELETE", "/api/v0/blueprints/delete/test", ``, http.StatusOK, `{"status":true}`},
+		{"DELETE", "/api/v0/blueprints/delete/test3-non", ``, http.StatusBadRequest, `{"status":false,"errors":[{"id":"BlueprintsError","msg":"Unknown blueprint: test3-non"}]}`},
 	}
 
 	for _, c := range cases {