core: Only catch errors from onBeforeUpload

Previously this code would treat any error at all as an `onBeforeUpload`
error, and show the informer. We also lost the stack trace for errors
because `err` is stringified in the handler. This is correct for
`onBeforeUpload` rejections, since they are string reasons, but not for
other errors.

Now only actual `onBeforeUpload` errors are handled that way, and fatal
upload errors end up in the StatusBar like before and in the console,
with a stack trace.

src/core/Core.js

@@ -669,7 +669,10 @@ class Uppy {
       return Promise.reject('Minimum number of files has not been reached')
     }
 
-    return this.opts.onBeforeUpload(this.getState().files).then(() => {
+    return this.opts.onBeforeUpload(this.getState().files).catch((err) => {
+      this.emit('informer', err, 'error', 5000)
+      return Promise.reject(`onBeforeUpload: ${err}`)
+    }).then(() => {
       this.emit('core:upload')
 
       const waitingFileIDs = []
@@ -700,10 +703,6 @@ class Uppy {
         this.emit('core:success', waitingFileIDs)
       })
     })
-    .catch((err) => {
-      this.emit('informer', err, 'error', 5000)
-      return Promise.reject(`onBeforeUpload: ${err}`)
-    })
   }
 }