  We present the first comprehensive look at the $0.35-5$ $\mu$m transmission
spectrum of the warm ($\sim 800$ K) Neptune HAT-P-11b derived from thirteen
individual transits observed using the Hubble and Spitzer Space Telescopes.
Along with the previously published molecular absorption feature in the
$1.1-1.7$ $\mu$m bandpass, we detect a distinct absorption feature at 1.15
$\mu$m and a weak feature at 0.95 $\mu$m, indicating the presence of water
and/or methane with a combined significance of 4.4 $\sigma$. We find that this
planet's nearly flat optical transmission spectrum and attenuated near-infrared
molecular absorption features are best-matched by models incorporating a
high-altitude cloud layer. Atmospheric retrievals using the combined $0.35-1.7$
$\mu$m HST transmission spectrum yield strong constraints on atmospheric
cloud-top pressure and metallicity, but we are unable to match the relatively
shallow Spitzer transit depths without under-predicting the strength of the
near-infrared molecular absorption bands. HAT-P-11b's HST transmission spectrum
is well-matched by predictions from our microphysical cloud models. Both
forward models and retrievals indicate that HAT-P-11b most likely has a
relatively low atmospheric metallicity ($<4.6 \; Z_{\odot}$ and $<86 \;
Z_{\odot}$ at the $2 \sigma$ and $3 \sigma$ levels respectively), in contrast
to the expected trend based on the solar system planets. Our work also
demonstrates that the wide wavelength coverage provided by the addition of the
HST STIS data is critical for making these inferences.
