Non-descriptive error messages when a package upload fails in com_installer

## Executive summary

When you try installing an extension package file with Manage Extensions but your PHP upload temporary directory (upload_tmp_dir) is not set in pph.ini or the package size is larger than the maximum upload size (max_upload_size) you get a generic error which doesn't let you understand why the upload failed. This PR adds descriptive error messages using the same language strings we are already using in the Warnings page.

## Backwards compatibility

No backwards compatibility issues are introduced.

## Translation impact

No translation strings are added, removed or modified.

## Testing instructions

Test 1: Make sure that your server's php.ini has a max_upload_size of 1M. Try installing an extension bigger than that, e.g. Admin Tools Core, K2, Community Builder etc. You get a generic upload error. Apply the PR and retry. You now get an error telling you that the upload failed and that your maximum upload limit is too small.

Test 2 (the problem may not appear on all servers): Make sure that your server's php.ini doesn't have an upload_tmp_dir set. Try installing any extension. You get a generic upload error. Apply the PR and retry. You now get an error telling you that the upload failed and the PHP temporary directory is not set.