empty variables lead to hard-to-find problems

[auerswald]

https://github.com/Neilpang/acme.sh/blob/e6cda79ee8411255d191f7781df42522bbf3aedc/acme.sh#L3578

the type-object in $response can contain other objects in case of an error. for example:

response='{"identifier":{"type":"dns","value":"example.com"},"status":"invalid","expires":"2018-01-19T01:31:55Z","challenges":[{"type":"dns-01","status":"invalid","error":{"type":"urn:ietf:params:acme:error:connection","detail":"DNS problem: NXDOMAIN looking up TXT for _acme-challenge.example.com","status": 400},"url":"https://acme-staging-v02.api.letsencrypt.org/acme/challenge/8m497...gfDiwc/91764091","token":"kKSZCs...Z9ung","keyAuthorization":"kKSZCs...Z9ung.HOF2...peL8"}],"wildcard": true}'

the regular expression '[^\{]*"type":"'$vtype'"[^\}]*' fails here. the variables token and uri are then empty. if you delete the error object here, the script works and later the (following) correct error appears: "Unable to update challenge :: The challenge is not pending."
this message helps a little bit more than an avoidable curl (3) error

example:

entry="$(printf "%s\n" "$response" | sed 's/,"error":{[^}]*}//g' | _egrep_o '[^\{]*"type":"'$vtype'"[^\}]*')"

I only took a quick look at the source code. Maybe you (can|must) catch the error before.
btw… thanks for your work!

[Neilpang]

I just fixed acme.sh to give a more friendly error message.
please update to the latest dev branch and try again.

export BRANCH=dev
acme.sh --upgrade