More detailed error 400's

[ZeroIntensity]

Right now, it just says "Bad Request". Some other information could probably be added (at least for debug mode).

[ZeroIntensity]

Example API, could integrate with #95

from view import new_app

app = new_app()

@app.get("/")
async def index():
    ...

@app.error(400)
@context
async def error(ctx: Context):
    ...

[ZeroIntensity]

Partially implemented by #137