feat: improve validate output

[GGabriele]

$ deck validate --online
Error: validate entity 'services (svc1)': HTTP status 400 (message: "schema violation (host: must not have a port)")
validate entity 'certificates (13c562a1-191c-4464-9b18-e5222b46035b)': HTTP status 400 (message: "2 schema violations (cert: invalid certificate: x509.new: asn1/tasn_dec.c:309:error:0D07803A:asn1 encoding routines:asn1_item_embed_d2i:nested asn1 error; key: invalid key: pkey.new:load_key: nil failed)")
validate entity 'plugins (prometheus)': HTTP status 400 (message: "schema violation (protocols.1: expected one of: grpc, grpcs, http, https, tcp, tls, tls_passthrough, udp)")
validate entity 'snis (demo1.example.com:123)': HTTP status 400 (message: "schema violation (name: must not have a port)")

[codecov-commenter]

Codecov Report

Merging #588 (3de604b) into main (299542a) will decrease coverage by 0.73%.
The diff coverage is 63.63%.

@@            Coverage Diff             @@
##             main     #588      +/-   ##
==========================================
- Coverage   51.15%   50.42%   -0.74%     
==========================================
  Files          73       74       +1     
  Lines        8009     8139     +130     
==========================================
+ Hits         4097     4104       +7     
- Misses       3542     3665     +123     
  Partials      370      370              

Impacted Files	Coverage Δ
validate/validate.go	5.38% <63.63%> (ø)

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[hbagdi]

An E2E integration test would help here. Please add an issue for it.