Invalid source map causes unchecked exception

If a source file contains a sourcemap with invalid encoding, then this code:

https://github.com/google/closure-compiler/blob/1f7d74b3c06c95c0264bc6722e6b1af17e08f7b4/src/com/google/javascript/jscomp/parsing/parser/Parser.java#L260

can throw an unchecked exception, see:

https://google.github.io/guava/releases/16.0/api/docs/com/google/common/io/BaseEncoding.html#decode(java.lang.CharSequence)

This should instead be caught and made into a compiler warning or error or something.