Make error message complaining about schema discovery support explicit about the missing `discover` capability
Hi, I added my custom tap as per the tutorial on https://meltano.com/tutorials/create-a-custom-extractor.html
```bash
../venv/meltano/bin/meltano add --custom extractor tap-fishtown-csv
(namespace): tap-fishtown-csv
(pip_url): git+https://gitlab.com/tjb-bnb/tap-csv.git
(executable): tap-csv
(capabilities): state
(settings): start_date,bucket,backend,tables
```
I can invoke it with the expected results:
```bash
../venv/meltano/bin/meltano invoke tap-fishtown-csv
```
But I get a stack trace if I continue with the tutorial:
```bash
$ ../venv/meltano/bin/meltano select --list tap-fishtown-csv '*' '*'
[2020-07-16 20:54:57,541] [8659|MainThread|root] [INFO] Catalog not found, trying to run the tap with --discover.
[2020-07-16 20:54:57,541] [8659|MainThread|root] [ERROR] Extractor 'tap-fishtown-csv' does not support schema discovery
Traceback (most recent call last):
File "/singer/venv/meltano/lib/python3.7/site-packages/meltano/cli/select.py", line 52, in select
show(project, extractor, show_all=flags["all"])
File "/singer/venv/meltano/lib/python3.7/site-packages/meltano/cli/select.py", line 99, in show
list_all = select_service.list_all(session)
File "/singer/venv/meltano/lib/python3.7/site-packages/meltano/core/select_service.py", line 46, in list_all
schema = self.load_schema(session)
File "/singer/venv/meltano/lib/python3.7/site-packages/meltano/core/select_service.py", line 33, in load_schema
self.extractor.run_discovery(invoker)
File "/singer/venv/meltano/lib/python3.7/site-packages/meltano/core/plugin/singer/tap.py", line 95, in run_discovery
f"Extractor '{self.name}' does not support schema discovery"
meltano.core.plugin.error.PluginLacksCapabilityError: Extractor 'tap-fishtown-csv' does not support schema discovery
Error: Cannot list the selected attributes: the tap does not support schema discovery or selection.
```
issue