ALTER TABLE behind-the-scenes effects' CONTEXT

Hi,

In the past I've found the error message in cases such as this somewhat
less helpful than it could be:

=# CREATE TABLE qqq (a int);
=# CREATE UNIQUE INDEX IF NOT EXISTS qqq_a_idx ON qqq(a);
=# ALTER TABLE qqq ALTER COLUMN a TYPE json USING NULL;
ERROR: data type json has no default operator class for access method
"btree"
HINT: You must specify an operator class for the index or define a
default operator class for the data type.

The attached patch adds a CONTEXT line to index and constraint rebuilds,
e.g:

CONTEXT: while rebuilding index qqq_a_idx

Any feedback welcome.

.m