Optionally continue on error in erc-auth-source-search

Optionally continue on error in erc-auth-source-search

* doc/misc/erc.texi (auth-source): Add new "Troubleshooting"
subsection.
* lisp/erc/erc.el (erc-open): Initialize markers before determining
session parameters.  Otherwise, functions that rely on
`erc-inset-marker' being non-nil, like `erc-check-text-conversion',
may fail during auth-source lookups.
(erc-auth-source-search): When non-interactive, ask the user whether
to continue connecting anyway.  (Bug#67978)