Make hex decoding consistent in RPC handlers.

The hex package requires an even number of characters in hex encoded
strings.  Some of the handlers already prepended a zero if necessary to
make this condition true, however other did not.  This commit extends this
functionality to all handlers and also makes the error return consistent.

ok @jrick
IMPORT PATH MODIFICATION CHANGESET
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Please use this changeset as the first one on top of the conformal codebase.
Note that if there are new imports, they need to be changed as part of this
changeset. In general, use "git rebase" on github.com/conformal's version
and apply this changeset first.