[lldb] Improve error message for modules with dots or dashes

LLDB does not like to import Python files with dashes or dots in their
name. While the former are technically allowed, it is discouraged. Dots
are allowed for subpackages but not in module names. This patch improves
the user experience by printing a useful error.

Before this patch:

  error: module importing failed: SyntaxError('invalid syntax',
  ('<string>', 1, 11, 'import foo-bar\n'))

After this patch:

  error: module importing failed: Python discourages dashes in module
  names: foo-bar

rdar://74263511

[1] https://www.python.org/dev/peps/pep-0008/#package-and-module-names

Differential revision: https://reviews.llvm.org/D96833