[Patch] Fix Unicode in errmsg

Hi All,

Just a small patch to fix server addition/connection errors when using
non-ascii languages (pt_BR in my case). I haven't delved enough into the
codebase to be sure, but there might be more occurrences of errmsg that
need decoding from ASCII to UTF-8.

Is there any plans to port PgAdmin4 to Python3, since Python 2 will only
be maintained until 2020? Python 3 support was added in Flask 0.10 and,
as mentioned in the Flask Docs, "Most libraries and Flask extensions
have been ported by now and using Flask with Python 3 is generally a
smooth ride.".

Thanks for this amazing piece of software.

--
Magnun Leno