ENH: linalg: Shorter ill-conditioned warning message

[pv]

Make the ill-conditioned matrix warning one-liner. Fix the missing
space between the number and text.

Previously, the warning read

__main__:1: LinAlgWarning: scipy.linalg.solve
Ill-conditioned matrix detected. Result is not guaranteed to be accurate.
Reciprocal condition number1.000000e-19

Some issues that are addressed here: there's no space before condition number, the message is quite long, usually the warnings don't contain the function name, and usually they don't contain newlines.

[ilayn]

Recip.number reads a bit odd, rcond might be better if we are going for an abbreviation. It is familiar enough from matlab. One of the tests is looking for an exact old phrase though.

[pv]

Ok, changed to rcond instead as it's shorter.
Initially I wanted to say "condition number" somewhere, but "ill-conditioned matrix" seems easily googleable.