Improve the reporting of D-Bus connection error

[imobachgs]

Improve the error reporting when a D-Bus connection cannot be established. By the way, it updates the changes file (I forgot to do it on #659).

Before:

Error: Connecting to Agama bus at unix:path=/run/agama/bus

Caused by:
    0: Connecting to Agama bus at unix:path=/run/agama/bus
    1: I/O error: Broken pipe (os error 32)
    2: Broken pipe (os error 32)

Now:

Could not connect to Agama bus at 'unix:path=/run/agama/bus'

Caused by:
    0: I/O error: Broken pipe (os error 32)
    1: Broken pipe (os error 32)

[coveralls]

coverage: 72.398%. remained the same when pulling 67cfefc on imobachgs:connection-error into f33d147 on openSUSE:master.

[joseivanlopez]

LGTM