reuse download --all doesn't work if you use SPDX-IDENTIFIER+

[carmenbianca]

To reproduce:

Create a file foo.py with:

# SPDX-License-Identifier: EUPL-1.2+

Then run reuse download --all:

Error: Failed to download license. 'EUPL-1.2+' is not a valid SPDX License Identifier.

Did you mean:
* EUPL-1.0
* EUPL-1.1
* EUPL-1.2

See <https://spdx.org/licenses/> for a list of valid SPDX License Identifiers.