chore(gov): improve proposal conversion error message (#15979)

CHANGELOG.md

@@ -69,6 +69,7 @@ Ref: https://keepachangelog.com/en/1.0.0/
 
 ### Improvements
 
+* (gov) [#15979](https://github.com/cosmos/cosmos-sdk/pull/15979) Improve gov error message when failing to convert v1 proposal to v1beta1.
 * (crypto) [#3129](https://github.com/cosmos/cosmos-sdk/pull/3129) New armor and keyring key derivation uses aead and encryption uses chacha20poly
 * (x/slashing) [#15580](https://github.com/cosmos/cosmos-sdk/pull/15580) Refactor the validator's missed block signing window to be a chunked bitmap instead of a "logical" bitmap, significantly reducing the storage footprint.
 * [#15448](https://github.com/cosmos/cosmos-sdk/pull/15448) Automatically populate the block timestamp for historical queries. In contexts where the block timestamp is needed for previous states, the timestamp will now be set. Note, when querying against a node it must be re-synced in order to be able to automatically populate the block timestamp. Otherwise, the block timestamp will be populated for heights going forward once upgraded.

x/gov/migrations/v3/convert.go

@@ -49,7 +49,7 @@ func ConvertToLegacyProposal(proposal v1.Proposal) (v1beta1.Proposal, error) {
 		return v1beta1.Proposal{}, err
 	}
 	if len(msgs) != 1 {
-		return v1beta1.Proposal{}, sdkerrors.ErrInvalidType.Wrap("can't convert a gov/v1 Proposal to gov/v1beta1 Proposal when amount of proposal messages is more than one")
+		return v1beta1.Proposal{}, sdkerrors.ErrInvalidType.Wrap("can't convert a gov/v1 Proposal to gov/v1beta1 Proposal when amount of proposal messages not exactly one")
 	}
 	if legacyMsg, ok := msgs[0].(*v1.MsgExecLegacyContent); ok {
 		// check that the content struct can be unmarshalled