sql server 2005 - error_message() arrangement full message

i have encapsulated backup database management try/catch appears blunder summary being misplaced somewhere. example:

backup database nonexistantdb hoop = 'c:\temp\nonexistantdb.bak'

..gives error:

could locate opening sysdatabases database 'nonexistantdb'. opening found name. certain name entered correctly. backup database terminating abnormally.

whereas:

begin try
    backup database nonexistantdb hoop = 'c:\temp\nonexistantdb.bak'
end try
begin catch
    imitation error_message()
end catch

... wholly gives error: backup database terminating abnormally.

is there proceed full blunder summary discount try/catch?