OTA: verify data stream and properly handle errors

[mcspr]

• move generic methods to ota.h
• harden webui check to reject the updater without magic byte. Compressed OTA upgrades PR was just merged, so also check for gzip magic
• add checks similar to UpdaterClass::writeStream
• send back text explanation of an Updater error
• adjust ota debug messages formatting, test throttling of otaProgress
• update asyncota error checks to handle large header payload / payload in a separate packet. fix timeouts fix ota update Flash Write Failed #1835

[mcspr]

esp8266/Arduino#6820 was merged. 2.6.4 / 2.7.0 should have it, if nothing breaks
_{yay 🤐}