fix(linux): check for existing file before trying to install

[ermshiperete]

Under some circumstances it's possible that the user manages to select a directory instead of a kmp file, or manually enters a non-existing file. Is used to throw an error that we caught on Sentry; the user was able to continue to use the app. This change now checks for a valid file before trying to install the kmp file.

Fixes: #15572
Fixes: KEYMAN-LINUX-99

User Testing

TEST_FIX:

• start km-config and click the "Install keyboard" button
• in the "Choose a kmp file" dialog press Ctrl+L to get an input field
• enter an invalid path, e.g. /homex, and click Open
• verify that the file dialog stays open until you enter or select a valid file or cancel the dialog

[keymanapp-test-bot]

User Test Results

Test specification and instructions

• ✅ TEST_FIX (PASSED) (notes)

[Meng-Heng]

Test Prerequisites

1. Ensure Keyman for Linux is on the right version by using km-package-list-installed --version in Terminal

Test Specs

1. macOS Intel, Virtual Box
2. Ubuntu Noble
3. km-package-list-installed version 19.0.201-alpha (package version 19.0.201-1~PR-15573)

Test Results

• TEST_FIX (PASSED):

1. In the Terminal, type km-config
2. In Keyman Configuration, click "Install keyboard"
3. In "Choose a kmp file", press CTRL + L to enter the input field
4. Enter /homex, click "Open"
5. Verify: The dialog remains open until clicking on a folder.