Onion neterror screenshots

Screenshots for tor-browser.ftl strings added in !95 (merged).

screenshots/onion-authentication-prompt.png is for:

• onion-neterror-authorization-title (the tab name).

screenshots/onion-neterror-authentication-failed.png is for:

• onion-neterror-page-title (the tab name).
• onion-neterror-incorrect-authentication-header
• onion-neterror-incorrect-authentication-description

screenshots/onion-neterror-authentication-required.png is for:

• onion-neterror-missing-authentication-header
• onion-neterror-missing-authentication-description

screenshots/onion-neterror-disconnected.png is for:

• onion-neterror-disconnected-header
• onion-neterror-disconnected-description

screenshots/onion-neterror-invalid.png is for:

• onion-neterror-invalid-address-header
• onion-neterror-invalid-address-description

screenshots/onion-neterror-no-connect.png is for:

• onion-neterror-connection-failed-header
• onion-neterror-connection-failed-description

screenshots/onion-neterror-not-found.png is for:

• onion-neterror-not-found-header
• onion-neterror-not-found-description

screenshots/onion-neterror-time-out.png is for:

• onion-neterror-timed-out-header
• onion-neterror-timed-out-description

screenshots/onion-neterror-unreachable.png is for:

• onion-neterror-unreachable-header
• onion-neterror-unreachable-description