add explicit info for ext-gmp or ext-bcmath and earlier exception handling if not available

[jpmschuler]

I ran into installing the extension, configuring my auth token, but not being able to login with "Verification error". I only found in the logs that ext-gmp or ext-bcmath are necessary. Installing ext-gmp resolved any issues and made the extension wonderfully zero-con.

I propose to

• add an info in the README about this
• add a composer.json suggests for ext-gmp perhaps
• check for ext-gmp or ext-bcmath before/during enabling the auth mechanism for a user (so early and where visible messages and exceptions are possible) instead of during first auth where only general logging is possible

(can supply PR if I circle back to this as soon as time comes up)