Better DNS error handling

[isidentical]

E.g

$ http non.existent.url
http: error: gaierror: [Errno -2] Name or service not known
Couldn't resolve the given URL. Perhaps check the URL and try again.

or

http: error: gaierror: [Errno -3] Temporary failure in name resolution
Couldn't connect to a DNS server. Perhaps check your connection and try again.

Fixes #1248

[codecov-commenter]

Codecov Report

Merging #1249 (def9f85) into master (4d7d6b6) will decrease coverage by 0.86%.
The diff coverage is 94.04%.

❗ Current head def9f85 differs from pull request most recent head e4da2fd. Consider uploading reports for the commit e4da2fd to get more accurate results

@@            Coverage Diff             @@
##           master    #1249      +/-   ##
==========================================
- Coverage   97.28%   96.42%   -0.87%     
==========================================
  Files          67       82      +15     
  Lines        4235     5593    +1358     
==========================================
+ Hits         4120     5393    +1273     
- Misses        115      200      +85     

Impacted Files	Coverage Δ
tests/test_binary.py	100.00% <ø> (ø)
httpie/compat.py	31.11% <27.90%> (-68.89%)	⬇️
httpie/output/lexers/http.py	52.00% <52.00%> (ø)
tests/conftest.py	77.14% <58.33%> (-9.82%)	⬇️
tests/test_ssl.py	91.01% <66.66%> (-3.93%)	⬇️
httpie/output/formatters/colors.py	89.05% <75.60%> (-4.53%)	⬇️
httpie/manager/__main__.py	82.35% <82.35%> (ø)
httpie/models.py	94.73% <83.33%> (-2.64%)	⬇️
httpie/uploads.py	98.68% <83.33%> (-1.32%)	⬇️
httpie/manager/core.py	92.85% <92.85%> (ø)
... and 56 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5a83a9e...e4da2fd. Read the comment docs.