fix: show warning for non-web targets

[snitin315]

trafficstars

• [x] This is a bugfix
• [ ] This is a feature
• [x] This is a code refactor
• [x] This is a test update
• [ ] This is a docs update
• [ ] This is a metadata update

For Bugs and Features; did you add new tests?

Yes

Motivation / Use-Case

fixes https://github.com/webpack/webpack-dev-server/pull/2104

Breaking Changes

None

Additional Info

None

[codecov[bot]]

Codecov Report

Merging #3094 (71a9eb9) into master (257e2eb) will increase coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3094      +/-   ##
==========================================
+ Coverage   95.69%   95.71%   +0.01%     
==========================================
  Files          34       34              
  Lines        1279     1284       +5     
  Branches      368      370       +2     
==========================================
+ Hits         1224     1229       +5     
  Misses         51       51              
  Partials        4        4              

Impacted Files	Coverage Δ
lib/Server.js	96.03% <100.00%> (ø)
lib/utils/DevServerPlugin.js	98.91% <100.00%> (+0.06%)	:arrow_up:

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[anshumanv]

need rebase