Fix pylint 2.6.0 raise-missing-from warning

[weiji14]

Description of proposed changes

Fixes failing style checks due to pylint 2.6.0 adding a new "raise-missing-from" check for exceptions that should have a cause. The try-except cause needs to be re-written as:

try:
    # do something
except Error as e:
    raise GMTInvalidInput("some better error message") from e

See also:

• https://docs.pylint.org/en/latest/whatsnew/2.6.html#new-checkers
• PEP 3134 -- Exception Chaining and Embedded Tracebacks
• https://blog.ram.rachum.com/post/621791438475296768/improving-python-exception-chaining-with

Fixes #

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.