Fix bug in HTTP server's form-data parsing

Did not handle a non-empty preamble properly
fixup! Improve Rhizome HTTP RESTful interface
Refactor Rhizome manifest validation

New function returns text describing the failed validation
fixup! fixup! Improve Rhizome HTTP RESTful interface
fixup! Rhizome Java API: list bundles
fixup! Rhizome Java API: get manifest
fixup! fixup! Rhizome Java API: list bundles
fixup! fixup! Rhizome Java API: get manifest
fixup! Rhizome Java API: get raw payload
fixup! Rhizome Java API: get decrypted payload
fixup! UNFINISHED