When using `lbrynet get URL`, if the URL is not a valid URL
the function `url.URL.parse` will raise a `ValueError` exception
which will produce a whole backtrace.
For example, this is the case if we provide a channel name
with a forward slash but without a stream name.
```
lbrynet get @Non-existing/
```
```
Traceback (most recent call last):
File "/opt/git/lbry-sdk/lbry/file/file_manager.py", line 84, in download_from_uri
if not URL.parse(uri).has_stream:
File "/opt/git/lbry-sdk/lbry/schema/url.py", line 114, in parse
raise ValueError('Invalid LBRY URL')
ValueError: Invalid LBRY URL
WARNING lbry.extras.daemon.daemon:1110: Error downloading Non-existing/: Invalid LBRY URL
```
Now we raise a new `InvalidStreamURLError` which can be trapped in the upper functions
that use `url.URL.parse` such as `FileManager.download_from_uri`.
If we do this the traceback won't be shown.
```
WARNING lbry.file.file_manager:252:
Failed to download Non-existing/: Invalid LBRY stream URL: '@Non-existing/'
WARNING lbry.extras.daemon.daemon:1110:
Error downloading Non-existing/: Invalid LBRY stream URL: '@Non-existing/'
```
This handles the case when trying to download only "channel" parts
without the claim part.
```
lbrynet get @Non-existing
lbrynet get @Non-existing/
lbrynet get Non-existing/
```
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