veth: throw better error when peer is missing

[ffmancera]

When creating a veth interface if the peer is missing, Nmstate is now
throwing a better error.

Signed-off-by: Fernando Fernandez Mancera ffmancera@riseup.net

[codecov]

Codecov Report

Merging #1692 (b53c419) into base (c98c8a4) will decrease coverage by 2.73%.
The diff coverage is 100.00%.

❗ Current head b53c419 differs from pull request most recent head 21acf1c. Consider uploading reports for the commit 21acf1c to get more accurate results

@@            Coverage Diff             @@
##             base    #1692      +/-   ##
==========================================
- Coverage   93.75%   91.01%   -2.74%     
==========================================
  Files          84       83       -1     
  Lines        7030     7024       -6     
==========================================
- Hits         6591     6393     -198     
- Misses        439      631     +192     

Impacted Files	Coverage Δ
libnmstate/ifaces/ifaces.py	92.57% <100.00%> (-4.37%)	⬇️
libnmstate/ifaces/ethtool.py	64.34% <0.00%> (-35.66%)	⬇️
libnmstate/nm/ethtool.py	59.67% <0.00%> (-35.49%)	⬇️
libnmstate/nm/wired.py	76.47% <0.00%> (-23.53%)	⬇️
libnmstate/ifaces/infiniband.py	76.66% <0.00%> (-20.00%)	⬇️
libnmstate/nm/translator.py	73.77% <0.00%> (-19.68%)	⬇️
libnmstate/validator.py	75.47% <0.00%> (-16.99%)	⬇️
libnmstate/ifaces/ethernet.py	85.71% <0.00%> (-10.72%)	⬇️
libnmstate/ifaces/macvlan.py	90.47% <0.00%> (-7.15%)	⬇️
libnmstate/route.py	91.13% <0.00%> (-6.97%)	⬇️
... and 18 more

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