Incorrect error message when some but not all readers found

[gerritholl]

Describe the bug

When creating a Scene object with multiple readers, and one or more of the readers are not found, the exception lists all requested readers as not found. This is confusing.

To Reproduce

import glob
from satpy import Scene
fn_sev = glob.glob("/media/nas/x21308/scratch/SEVIRI/2013/12/12/*")
fn_nok = ["/grapefruit/not/found"]
reader_sev = "seviri_l1b_hrit"
reader_nok = "fruit-reader"
sc = Scene(filenames={reader_sev: fn_sev})  # ok
sc = Scene(filenames={reader_sev: fn_sev, reader_nok: fn_nok})

Expected behavior

I expect the exception:

Traceback (most recent call last):
  File "mwe46.py", line 8, in <module>
    sc = Scene(filenames={reader_sev: fn_sev, reader_nok: fn_nok})
  File "/data/gholl/miniconda3/envs/py38/lib/python3.8/site-packages/satpy/scene.py", line 149, in __init__
    self.readers = self.create_reader_instances(filenames=filenames,
  File "/data/gholl/miniconda3/envs/py38/lib/python3.8/site-packages/satpy/scene.py", line 193, in create_reader_instances
    return load_readers(filenames=filenames,
  File "/data/gholl/miniconda3/envs/py38/lib/python3.8/site-packages/satpy/readers/__init__.py", line 706, in load_readers
    for idx, reader_configs in enumerate(configs_for_reader(reader, ppp_config_dir)):
  File "/data/gholl/miniconda3/envs/py38/lib/python3.8/site-packages/satpy/readers/__init__.py", line 552, in configs_for_reader
    raise ValueError("No reader(s) named: {}".format(reader))
ValueError: No reader(s) named: ['fruit-reader']

Actual results

In reality I get the exception:

Traceback (most recent call last):
  File "mwe46.py", line 8, in <module>
    sc = Scene(filenames={reader_sev: fn_sev, reader_nok: fn_nok})
  File "/data/gholl/miniconda3/envs/py38/lib/python3.8/site-packages/satpy/scene.py", line 149, in __init__
    self.readers = self.create_reader_instances(filenames=filenames,
  File "/data/gholl/miniconda3/envs/py38/lib/python3.8/site-packages/satpy/scene.py", line 193, in create_reader_instances
    return load_readers(filenames=filenames,
  File "/data/gholl/miniconda3/envs/py38/lib/python3.8/site-packages/satpy/readers/__init__.py", line 706, in load_readers
    for idx, reader_configs in enumerate(configs_for_reader(reader, ppp_config_dir)):
  File "/data/gholl/miniconda3/envs/py38/lib/python3.8/site-packages/satpy/readers/__init__.py", line 552, in configs_for_reader
    raise ValueError("No reader(s) named: {}".format(reader))
ValueError: No reader(s) named: ['seviri_l1b_hrit', 'fruit-reader']

It claims that neither of the readers are found. This is not true. Either it should only list the readers that are not found, or the error message should be reformulated to something like "At least one of the following readers could not be found:". The former would be more helpful.

Environment Info:

• OS: openSUSE 15.0
• Satpy Version: 0.21.1.dev79+g493ac55f