Make error message wording consistent (closes #165)

Error messages in mfer/ and internal/cli/ are lowercase except names and
acronyms, carry no "failed to" or command-name prefix, and each wrap names
only the operation and thing the wrapped error does not already name, so a
stacked message names what failed once. Wraps around errors that already
name their operation and path (os and afero path errors, url.Error, the
builder's path errors, the gpg helpers' own errors) are dropped. gpg's
stderr is appended to a gpg failure, and to the error for a signing key gpg
did not report, only when gpg wrote some. errHTTPStatus reads "unexpected
HTTP status"; both inner-not-set sentinels read "inner message not set".
No sentinel, errors.Is result or exit status changes.

Model: opus-5-5

mfer/gpg_test.go

@@ -630,17 +630,6 @@ func TestGPGSignKeyNotReportedKeepsStderr(t *testing.T) {
 			"gpg: note from the fake gpg")
 }
 
-// TestGPGFailureKeepsStderr puts a fake gpg first on PATH that writes to
-// stderr and exits non-zero, and checks that what it wrote ends the message.
-func TestGPGFailureKeepsStderr(t *testing.T) {
-	t.Setenv("PATH", fakeGPGPath(t,
-		"#!/bin/sh\necho 'gpg: signing failed: No secret key' >&2\nexit 2\n"))
-
-	_, _, err := gpgSign(context.Background(), []byte("data"), GPGKeyID("any"))
-	assert.EqualError(t, err,
-		"gpg sign: exit status 2: gpg: signing failed: No secret key")
-}
-
 // TestGPGCancelWhenChildHoldsOutput uses a fake gpg that runs sleep as a
 // child instead of exec-ing it, the way a wrapper script around the real
 // gpg might. Killing the fake gpg leaves sleep holding its stdout and