amcheck: Improve error message for partitioned index target.HEADmaster

Previously, amcheck could produce misleading error message when
a partitioned index was passed to functions like bt_index_check().
For example, bt_index_check() with a partitioned btree index produced:

    ERROR:  expected "btree" index as targets for verification
    DETAIL:  Relation ... is a btree index.

Reporting "expected btree index as targets" even when the specified
index was a btree was confusing. In this case, the function should fail
since the partitioned index specified is not valid target. This commit
improves the error reporting to better reflect this actual issue. Now,
bt_index_check() with a partitioned index, the error message is:

    ERROR:  expected index as targets for verification
    DETAIL:  This operation is not supported for partitioned indexes.

This commit also applies the following minor changes:

- Simplifies index_checkable() by using get_am_name() to retrieve
   the access method name.

- Changes index_checkable() from extern to static, as it is only used
   in verify_common.c.

- Updates the error code for invalid indexes to
   ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE,
   aligning with usage in similar modules like pgstattuple.

Author: Masahiro Ikeda <ikedamsh@oss.nttdata.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/8829854bbfc8635ddecd0846bb72dfda@oss.nttdata.com

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