Fix 28 gcc output parse error

Fix #28:

1. Modified gcc output parser to output an error for the case mentioned in the issue.
2. Updated/fixed the linker test in the parser spec:
The test expected one error. However, it should have expected two errors, but the parser returned only one because the gcc output string used a file name other than `program.c`. If the test were updated to use `program.c`, the test would have broken. Now, after updating the parser regex to take into account file names other than `program.c`, the test broke. So, I fixed the test to expect two errors and modified the test input to use `program.c` anyway.

3. Just as a fallback, if the compilation fails but the parser fails to return any errors, the error label shows a generic message instead of the error/warning count. Previously, the error label was empty, so the user couldn't click on it to reveal the gcc output window.
4. `index.html` cleanup