counters ipc: handle single-task dumps correctly

[hawkw]

Currently, humility counters ipc will die if run against a dump that only contains a single task. This is because because it attempts to load the task table from the kernel in order to read task generations/restart counts, and returns an error if the task table can't be loaded.

This branch changes humility counters ipc to handle single-task dumps more gracefully. Now, if we can't load the task table, we just print a warning and continue on with an empty task table. We won't print generations or restart counts in this case, since there's no way to determine that from a dump of a single task, but we will still show the counters, which seems strictly better than the current behavior.

For example:

$ cargo run -- -d tests/cmd/cores/hubris.core.u16-ringbuf counters ipc
   Compiling humility-cmd-counters v0.1.0 (/home/eliza/Code/oxide/humility/cmd/counters)
   Compiling humility v0.11.4 (/home/eliza/Code/oxide/humility)
    Finished dev [unoptimized + debuginfo] target(s) in 6.46s
     Running `target/debug/humility -d tests/cmd/cores/hubris.core.u16-ringbuf counters ipc`
humility: attached to dump
humility: WARNING: failed to load task table: read of 4704 bytes from invalid address: 0x24000490
humility: WARNING: no generations/restart counts will be displayed.
humility: note: this may be a single-task dump.
drv_spi_api::__SPI_CLIENT_COUNTERS
 fn Spi::write() .................................................... 530 calls
    clients:
    task gimlet_seq .................................... = 0 ......... = 530 ok

 fn Spi::exchange() ................................................... 5 calls
    clients:
    task gimlet_seq .................................... = 0 ........... = 5 ok

 fn Spi::lock() ....................................................... 4 calls
    clients:
    task gimlet_seq .................................... = 0 ........... = 4 ok

 fn Spi::release() .................................................... 1 calls
    clients:
    task gimlet_seq .................................... = 0 ........... = 1 ok

drv_stm32xx_sys_api::__SYS_CLIENT_COUNTERS
 fn Sys::gpio_configure_raw() ........................................ 12 calls
    clients:
    task gimlet_seq .................................... = 0 .......... = 12 ok

 fn Sys::gpio_set_reset() ............................................ 12 calls
    clients:
    task gimlet_seq .................................... = 0 .......... = 12 ok

 fn Sys::gpio_read_input() ............................................ 5 calls
    clients:
    task gimlet_seq .................................... = 0 ........... = 5 ok

task_jefe_api::__JEFE_CLIENT_COUNTERS
 fn Jefe::set_state() ................................................. 1 calls
    clients:
    task gimlet_seq .................................... = 0 ........... = 1 ok

task_packrat_api::__PACKRAT_CLIENT_COUNTERS
 fn Packrat::set_spd_eeprom() ........................................ 32 calls
    clients:
    task gimlet_seq .................................... = 0 .......... = 32 ok

 fn Packrat::set_mac_address_block() .................................. 1 calls
    clients:
    task gimlet_seq .................................... = 0 ........... = 1 ok

 fn Packrat::set_identity() ........................................... 1 calls
    clients:
    task gimlet_seq .................................... = 0 ........... = 1 ok

This branch depends on @mkeeter's PR #476, because that branch added a failing dump to the test suite, and I wanted to update the test output from that dump.