Bug #691587: Catch scheme exceptions when converting error messages to a C string.

When loading a scheme data file such as ~/.gnucash/stylesheets that contains
invalid characters (e.g., all-zero bytes), the loading throws a scheme
exception and we are redirected into the error catch handler. When trying
to convert the erroneous scheme expression into a string for the
message output, scheme runs into an exception right again. Hence, the
conversion to string must be guarded by a stack_catch as well.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23059 57a11ea4-9604-0410-9ed3-97b8803252fd