curl: exit on config file parser errors

Like when trying to import an environment variable that does not exist.

Also fix a bug for reading env variables when there is a default value
set.

Bug: https://curl.se/mail/archive-2024-02/0008.html
Reported-by: Brett Buddin

Add test 462 to verify.

Closes #12862

tests/data/Makefile.inc

@@ -73,6 +73,7 @@ test426 test427 test428 test429 test430 test431 test432 test433 test434 \
 test435 test436 test437 test438 test439 test440 test441 test442 test443 \
 test444 test445 test446 test447 test448 test449 test450 test451 test452 \
 test453 test454 test455 test456 test457 test458 test459 test460 test461 \
+test462 \
 \
 test490 test491 test492 test493 test494 test495 test496 test497 test498 \
 test499 test500 test501 test502 test503 test504 test505 test506 test507 \

tests/data/test462

@@ -0,0 +1,39 @@
+<testcase>
+<info>
+<keywords>
+variables
+--config
+</keywords>
+</info>
+
+#
+# Server-side
+<reply>
+</reply>
+
+#
+# Client-side
+<client>
+<server>
+none
+</server>
+<name>
+Missing environment variables in config file
+</name>
+<file name="%LOGDIR/cmd">
+variable %MISSING
+expand-data {{MISSING}}
+</file>
+<command>
+http://%HOSTIP:%HTTPPORT/%TESTNUMBER -K %LOGDIR/cmd
+</command>
+</client>
+
+#
+# Verify data after the test has been "shot"
+<verify>
+<errorcode>
+26
+</errorcode>
+</verify>
+</testcase>