Move exception handling up the stack (avoid `exit(1)` in our composable
functions)
Relevant to Synapse Pro for small hosts as we don't want to exit the
entire Python process and affect all homeserver tenants.
### Background
As part of Element's plan to support a light form of vhosting (virtual
host) (multiple instances of Synapse in the same Python process) (c.f
Synapse Pro for small hosts), we're currently diving into the details
and implications of running multiple instances of Synapse in the same
Python process.
"Clean tenant provisioning" tracked internally by
https://github.com/element-hq/synapse-small-hosts/issues/48
e00a411837