prefeature: guide use of error propagation

[bjacobgordon]

Steps

• refactor(dx): guides propagation of "actionable" errors #226
• refactor(dx): guides propagation of "non-actionable" errors #227
• refactor(dx): enforces use of guided error propagation #228
• fix(text-to-image): ensures that "failed to fetch" is properly caught in SDXL client #236
• fix(library/Attempt): prevents nested NonActionableError instances #346