Explicitly mark HttpConnectError as a marker trait
Explicitly mark HttpConnectError as a marker trait

Although the `MarkerTrait` has become deprecated since the beta's
release, this change makes `solicit` fully compliant with the current
beta state.

Galvanization/solicit,gwicke/solicit,tempbottle/solicit,stepancheg/solicit,mlalic/solicit,jinnjuice/solicit