Fixed bug where webpage opens on bad paths

[sushimon]

Motivation and Context

Calls to check_all and check_errors generate an error report when we pass in a path that doesn't exist or is unreadable.

Your Changes

Description:

• PythonTA no longer opens a webpage under these conditions but still prints the error message to the console.
• Added some logic to ensure at least one file has been checked before generating reports.
• Updated CHANGELOG.md to document this bug fix.

Type of change (select all that apply):

• Bug fix (non-breaking change which fixes an issue)

Testing

• Ran the tests in the test suite and verified they passed.
• Ran python in pyta repository and called check_all and check_errors (in reality, they both use the same helper method) with erroneous inputs (both files and directories) and verified that an error message was displayed but no webpage opened. Also tested with different PythonTA reporters.
• Also tested the check_all and check_errors methods with valid inputs and verified this change did not affect the messages being displayed (e.g. removing some by accident).
• Previewed the CHANGELOG.md file to verify it now includes information related to this bug fix.

Questions and Comments (if applicable)

• As of now, the PR for the ForbiddenPythonSyntaxChecker hasn't been merged so once it does, there will might be a merge conflict with the CHANGELOG.md files so that needs to be dealt with

Checklist

• I have performed a self-review of my own code.
• I have verified that the CI tests have passed.
• I have reviewed the test coverage changes reported on Coveralls.
• I have updated the CHANGELOG.md file.