Don't attempt el-doc without a session

When there is no session available, and eldoc was activated, there
would be an error message reported by eldoc complaining of a wrong
type.  The error was being caused by there being no tooling session.
Merge pull request #364 from hugoduncan/feature/dont-try-eldoc-without-session

Don't attempt el-doc without a session
Bump dash.el dep to 2.1.0
Fix library version discovery
Leverage function from dash 2.1 -elem-index
Fix next-error

Broken by nrepl-extract-error-info not returning the message as the
note.  This change also uses nrepl-note-p to locate notes, rather than
nrepl-note.
Merge pull request #365 from hugoduncan/feature/fix-next-error

Fix next-error