[Network Conditions]: Fixing vertical spacing bug caused by error text
[Network Conditions]: Fixing vertical spacing bug caused by error text

Issue:
- window scrolls up/down though 'User agent' text field is not empty and no error message is being shown

Changes:
- Error text isn't re-set to an empty string, but is hidden/revealed as needed

GIFs:
Broken: https://imgur.com/ocufEEF
Fixed: https://imgur.com/LmTAYqY

Bug: 1159319
Change-Id: Ia2945e9562641f2dd78bd46c951626c0523150f1
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2602551
Reviewed-by: Benedikt Meurer <024a1e0af501338d11c3b837c6193d11c140d792@chromium.org>
Reviewed-by: Patrick Brosset <f0b1693d3ec18fb190dd6deddb896f34ba3fd693@microsoft.com>
Commit-Queue: Michael Liao <00d75d81082fdf74688ec453c516b42f7cbf0f40@microsoft.com>

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