Simplify unknown-extension error message

[maddenp-noaa]

Synopsis

When a file with an unrecognized extension is specified, uwtools provides information on the underlying KeyError that is not useful to users. This PR simplifies the information presented to the user.

Old behavior:

>>> template.render(input_file="template.jinja2", values_src="values.txt")
Cannot deduce format of 'values.txt' from unknown extension 'txt'
Traceback (most recent call last):
  File "/home/maddenp/git/uwtools/src/uwtools/utils/file.py", line 54, in get_file_format
    return FORMAT.formats()[suffix]
           ~~~~~~~~~~~~~~~~^^^^^^^^
KeyError: 'txt'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/maddenp/git/uwtools/src/uwtools/api/template.py", line 50, in render
    result = _render(
             ^^^^^^^^
  File "/home/maddenp/git/uwtools/src/uwtools/config/jinja2.py", line 176, in render
    values = _supplement_values(
             ^^^^^^^^^^^^^^^^^^^
  File "/home/maddenp/git/uwtools/src/uwtools/config/jinja2.py", line 349, in _supplement_values
    values_format = values_format or get_file_format(values_src)
                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/maddenp/git/uwtools/src/uwtools/utils/file.py", line 58, in get_file_format
    raise UWError(msg) from e
uwtools.exceptions.UWError: Cannot deduce format of 'values.txt' from unknown extension 'txt'

New behavior:

>>> template.render(input_file="template.jinja2", values_src="values.txt")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/maddenp/git/uwtools/src/uwtools/api/template.py", line 50, in render
    result = _render(
             ^^^^^^^^
  File "/home/maddenp/git/uwtools/src/uwtools/config/jinja2.py", line 176, in render
    values = _supplement_values(
             ^^^^^^^^^^^^^^^^^^^
  File "/home/maddenp/git/uwtools/src/uwtools/config/jinja2.py", line 349, in _supplement_values
    values_format = values_format or get_file_format(values_src)
                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/maddenp/git/uwtools/src/uwtools/utils/file.py", line 56, in get_file_format
    raise UWError(msg) from None
uwtools.exceptions.UWError: Cannot deduce format of 'values.txt' from unknown extension 'txt'

CLI behavior remains:

$ echo "{{ x }}" | uw template render --values-file values.txt
Cannot deduce format of 'values.txt' from unknown extension 'txt'

Type

• Code maintenance (refactoring, etc. without behavior change)

Impact

• This is a non-breaking change (existing functionality continues to work as expected)

Checklist

• I have added myself and any co-authors to the PR's Assignees list.
• I have reviewed the documentation and have made any updates necessitated by this change.