Fix validation error of protocol not beeing returned

[glasgio]

Fixes #830

Fix the "https: or mailto:" error being overwritten by the error in the catch by passing it upwards trough the cause property

[marco-c]

Thanks!

[marco-c]

Oops, there is still some error:

/home/runner/work/web-push/web-push/src/vapid-helper.js
Error: 79:11 error 'subjectParseResult' is assigned a value but never used no-unused-vars
Error: 83:39 error 'subjectParseResult' is not defined no-undef
Error: 86:7 error 'subjectParseResult' is not defined no-undef

[glasgio]

Oops, there is still some error:

/home/runner/work/web-push/web-push/src/vapid-helper.js
Error: 79:11 error 'subjectParseResult' is assigned a value but never used no-unused-vars
Error: 83:39 error 'subjectParseResult' is not defined no-undef
Error: 86:7 error 'subjectParseResult' is not defined no-undef

It makes sense since the subjectParseResult is now scoped, sorry. Do you prefer an approach like defining the variable before the try?

let subjectParseResult = subject
try {
subjectParseResult = new URL(subject);

[marco-c]

Yeah, let's do:

let subjectParseResult = null;
try {
    subjectParseResult = new URL(subject);