Capturing the result status of an ALTER command in plpgsql

How does one capture the result status of an ALTER command inside
plpgsql code block?

For example:

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CREATE OR REPLACE FUNCTION public."Setup_Primary_Keys"()
RETURNS SETOF varchar AS
'DECLARE

BEGIN

ALTER TABLE public."HCM00101"  ADD CONSTRAINT "hcm00101-pk" PRIMARY
KEY("CUSTNMBR");
-- Capture Success or Failure here of the ALTER command
IF [how do I determine success here?] THEN
return next \'HCM00101 primary key set\';
ELSE [eat the failure exception here and continue]
return next \'HCM00101 primary key was not set, continuing with more
code below\';
END IF;

....... more code ........
return;

END;'
  LANGUAGE 'plpgsql' VOLATILE;
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