Update hesokuri.source-def namespace docstring

Docstring did not mention :live-edit-branches entry, even though it should.
Move validation logic of source-def/kind into a separate function

New function is source-def/validation-error, which returns a description of the
problem if there is one, or nil if there is none. This allows the heso object to
explicitly check if the configuration is valid, rather than allowing an
exception to occur if it is not.

Checks both the :live-edit-branches (if present) and :host-to-path entries.
Make heso object cautiously check source-defs for error

... rather than initialize a new heso object from the config file and possibly
cause an exception.
Replace uses of java.io.File constructor with clojure.java.io/file

The later is more flexible because it is varargs and allows strings or Files for
each argument.