Do not show full error message for missing weights

[danielhuppmann]

While working on the ECEMF-workflow migration, I realized that showing the full log message from the "missing-weights" error from pyam was really too much in the nomenclature output...

[phackstock]

Looks good to me in principle but if I remember correctly I put the full error there so I would get the complete output of all the missing values making it easier for debug.
I can of course also see the need for just getting a compact list.

[danielhuppmann]

But the error message only shows the first few lines anyway, and in this way, it's not really helpful...

[phackstock]

Ah I see, not super useful indeed if there's lots of things missing. I'd still want a way to know exactly what's missing though. Maybe something like this:

missing for
WITCH 5.4, DIAG-BASE, CANADA, [2005, 2010, 2015, 2020]
WITCH 5.4, DIAG-BASE, BRAZIL, [2005, 2010, 2015, 2020]
...

where the years are compacted

[danielhuppmann]

We can think of a more useful way for reporting missing weights later, merging this now for more readable log-output for now.