apiCommand(or upstream) needs to fail in case of failed invocation with proper info

[baranowb]

There is no sanity check here, this can lead to malformed outcome:

"
$ pr-merge upstream 4785
The hub commandline tool is not installed. Using curl.
Merging 4785 from upstream onto 7.4.x-proposed
The hub commandline tool is not installed. Using curl.

Retrieving PR information from https://api.github.com/repos/jbossas/jboss-eap7/pulls/4785... Done.
Git commit message will be: Merge pull request #4785 from
Continue...^C
"

With success:
"
$ pr-merge upstream 4785
The hub commandline tool is not installed. Using curl.
Merging 4785 from upstream onto 7.4.x-proposed
The hub commandline tool is not installed. Using curl.

Retrieving PR information from https://api.github.com/repos/jbossas/jboss-eap7/pulls/4785... Done.
Git commit message will be: Merge pull request #4785 from istudens:JBEAP-28731 [WFLY-20102] only encode and log route for first getRoute call
Continue...

"

[baranowb]

This is somewhat doubling #34

[baranowb]

curl:
-S, --show-error Show error even when -s is used
-s, --silent Silent mode

?