KeePass2Reader: fix error message logic

[c4rlo]

Bug found via lgtm.com.

Type of change

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

Description and Context

Fix error message when attempting to open a KeePass 1 database.

While I was at it, I also introduced a dedicated error message for I/O errors when attempting to read the file signature (though this is secondary and doesn't matter much).

Testing strategy

make test continues to pass.

Other than that, I've tested this manually, by attempting to open files /dev/null, CHANGELOG, and tests/data/basic.kdb.

Before my change

The error message in all three cases is:

Unable to open the database:
Error while reading the database: Not a KeePass database.

After my change

For /dev/null:

Unable to open the database:
Error while reading the database: Failed to read database file.

For CHANGELOG:

Unable to open the database:
Error while reading the database: Not a KeePass database.

For tests/data/basic.kdb:

Unable to open the database:
Error while reading the database: The selected file is an old KeePass 1 database (.kdb).

You can import it by clicking on Database > 'Import KeePass 1 database...'.
This is a one-way migration. You won't be able to open the imported database with the old KeePassX 0.4 version.

Checklist:

• ✅ I have read the CONTRIBUTING document. [REQUIRED]
• ✅ My code follows the code style of this project. [REQUIRED]
• ✅ All new and existing tests passed. [REQUIRED]
• ✅ I have compiled and verified my code with -DWITH_ASAN=ON. [REQUIRED]

[droidmonkey]

Can you create a database that matches the signature1/signature2 conditions to elicit the import error message?

[c4rlo]

Like my comment says above, tests/data/basic.kdb is such a database.

[droidmonkey]

I need to get my (non-existent) glasses checked! I glossed right over that last part of your PR.

[phoerious]

Wait the tests did pass before... Shouldn't they have failed of this is an issue?

[droidmonkey]

I think the issue is that the specific error message about being a KeePass1 database that must be imported was not being displayed. The parser correctly failed, just did not display the right error for the human.

[c4rlo]

That's correct.

[phoerious]

I see. Go on then.