Fix variable validation to show only one alert

[felixerdy]

Follow up of #465

[github-actions]

Tests

Test Name	Status	Flaky	Duration
Blockly Editor Page Tests [Blockly] visits the editor page	✅		968ms
Blockly Editor Page Tests [Blockly] visits the tutorial page	✅		174ms
Blockly Editor Page Tests [Blockly] visits the gallery page	✅		134ms
Blockly Editor Page Tests [Blockly] visits the faq page	✅		121ms
Blockly Editor Page Tests [Blockly] visits the settings page	✅		120ms
Blockly Editor Page Tests [Blockly] visits the login page	✅		121ms
Blockly Editor Page Tests [Blockly] navigates to tutorial and back	❌		14.0s
Blockly Editor Page Tests [Blockly] selects senseBox ESP	✅		719ms
Blockly Editor Page Tests [Blockly] selects senseBox MCU	✅		724ms
Blockly Editor Page Tests [Blockly] selects senseBox Mini	✅		683ms
Blockly Editor Page Tests [Blockly] compiles code	✅		19.9s
Code Editor Page Tests [CodeEditor] visits the code editor page	✅		1.3s
Code Editor Page Tests [CodeEditor] selects senseBox ESP	✅		457ms
Code Editor Page Tests [CodeEditor] compiles code	✅		615ms
Code Editor Page Tests [CodeEditor] opens reset dialog	✅		601ms
Code Editor Page Tests [CodeEditor] fetches libraries	✅		2.8s

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[mariopesch]

@felixerdy @georgi-s i think this PR superseeds this one with a simpler solution: #467