[webaudio] Return Err instead of panicking on input device requests

Currently we are yet to implement input stream support for CPAL's
webaudio host. Instead of panicking, we should return an
error, None or empty iterator in order to let the user write well
behaved cross-platform apps and notify the user accordingly rather than
crashing.

src/host/webaudio/mod.rs

@@ -85,7 +85,8 @@ impl Device {
     fn supported_input_configs(
         &self,
     ) -> Result<SupportedInputConfigs, SupportedStreamConfigsError> {
-        unimplemented!();
+        // TODO
+        Ok(Vec::new().into_iter())
     }
 
     #[inline]
@@ -105,7 +106,8 @@ impl Device {
 
     #[inline]
     fn default_input_config(&self) -> Result<SupportedStreamConfig, DefaultStreamConfigError> {
-        unimplemented!();
+        // TODO
+        Err(DefaultStreamConfigError::StreamTypeNotSupported)
     }
 
     #[inline]
@@ -167,7 +169,8 @@ impl DeviceTrait for Device {
         D: FnMut(&Data, &InputCallbackInfo) + Send + 'static,
         E: FnMut(StreamError) + Send + 'static,
     {
-        unimplemented!()
+        // TODO
+        Err(BuildStreamError::StreamConfigNotSupported)
     }
 
     /// Create an output stream.
@@ -406,7 +409,8 @@ impl Iterator for Devices {
 
 #[inline]
 fn default_input_device() -> Option<Device> {
-    unimplemented!();
+    // TODO
+    None
 }
 
 #[inline]