Make action mnemonic mandatory

Many actions don't set a mnemonic, which either complicates debugging or makes it impossible; while the progress message may provide comparable information, it's intended for human consumption, and often adds a lot of unnecessary overhead to logs and other instrumentation.

For example, in 3b0b6ed we're omitting progress messages from the execution log due to size concerns. At Google, we've been encouraging rule authors to always set the mnemonic.

We should consider making the mnemonic mandatory; we would have to do so behind a flag, at least initially.