go: return a bad magic error in check when parsing summary (foxglove#…

…1206)

### Changelog
- Changed: when attempting to read a corrupt MCAP using using the index,
the reader will return an ErrBadMagic instance rather than a bare Error.

### Docs

None.

### Description

it's useful to be able to tell this error apart from errors that might
result from i/o, but right now there is no simple way to determine this
using `errors.Is`. This PR gives the error a publicly known type, so
that callers can check for it.

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go/mcap/indexed_message_iterator.go

@@ -83,7 +83,7 @@ func (it *indexedMessageIterator) parseSummarySection() error {
 	}
 	magic := buf[20:]
 	if !bytes.Equal(magic, Magic) {
-		return fmt.Errorf("not an MCAP file")
+		return &ErrBadMagic{location: magicLocationEnd, actual: magic}
 	}
 	footer, err := ParseFooter(buf[:20])
 	if err != nil {

go/mcap/version.go

@@ -1,4 +1,4 @@
 package mcap
 
 // Version of the MCAP library.
-var Version = "v1.4.1"
+var Version = "v1.5.0"