improve plugin error formatting

Remove "checksum" from the error, and only indicate that the plugin has
changed.

Always show requested versions even if it's "any", and found versions of
plugins.

command/plugins.go

@@ -46,7 +46,7 @@ func (r *multiVersionProviderResolver) ResolveProviders(
 	var errs []error
 
 	chosen := choosePlugins(r.Available, reqd)
-	for name := range reqd {
+	for name, req := range reqd {
 		if newest, available := chosen[name]; available {
 			digest, err := newest.SHA256()
 			if err != nil {
@@ -54,7 +54,7 @@ func (r *multiVersionProviderResolver) ResolveProviders(
 				continue
 			}
 			if !reqd[name].AcceptsSHA256(digest) {
-				errs = append(errs, fmt.Errorf("provider.%s: checksum mismatch", name))
+				errs = append(errs, fmt.Errorf("provider.%s: new or changed plugin executable", name))
 				continue
 			}
 
@@ -63,20 +63,24 @@ func (r *multiVersionProviderResolver) ResolveProviders(
 		} else {
 			msg := fmt.Sprintf("provider.%s: no suitable version installed", name)
 
-			if req := reqd[name]; req.Versions.String() != "" {
-				foundVersions := []string{}
-				for meta := range r.Available.WithName(name) {
-					foundVersions = append(foundVersions, fmt.Sprintf("%q", meta.Version))
-				}
-
-				found := "none"
-				if len(foundVersions) > 0 {
-					found = strings.Join(foundVersions, ", ")
-				}
-
-				msg += fmt.Sprintf("\n  version requirements: %q\n  versions found: %s", req.Versions, found)
+			required := req.Versions.String()
+			// no version is unconstrained
+			if required == "" {
+				required = "(any version)"
 			}
 
+			foundVersions := []string{}
+			for meta := range r.Available.WithName(name) {
+				foundVersions = append(foundVersions, fmt.Sprintf("%q", meta.Version))
+			}
+
+			found := "none"
+			if len(foundVersions) > 0 {
+				found = strings.Join(foundVersions, ", ")
+			}
+
+			msg += fmt.Sprintf("\n  version requirements: %q\n  versions installed: %s", required, found)
+
 			errs = append(errs, errors.New(msg))
 		}
 	}